builder: declare Darwin socket and process symbols

[yohimik]

Scope

Add the libSystem declarations needed for Darwin sockets and posix_spawn. The minimal SDK does not provide these symbols. The builder assembles a second stub object and links it with the SDK object.

This branch now contains only the builder change and a direct socket test. The copied fcntl, process, weak-pointer and TLS commits have been removed. The shared builder support moved here from #5634 so that it has one owner. #5634 will depend on this PR instead of carrying a second copy.

This change does not need a net submodule update. A program can call the standard syscall socket API directly. Full netdev support is a separate dependency of tinygo-org/net#74.

The posix_spawn_file_actions_addchdir_np symbol requires macOS 10.15 or later when used by the process implementation.

Verification

The new host-only test testdata/darwinsockets.go creates a loopback listener with the syscall package, obtains its assigned port, connects, accepts, checks the peer address and shuts down the connection. It does not use the TinyGo netdev or require an external service.

The test passed with ordinary Go on native Darwin arm64. CI for the new TinyGo branch is pending. The earlier 24-check result was for the old combined branch and is not a result for this head.

The released fork already uses the same extra-stub mechanism for its socket and process probes. That is integration evidence, not a substitute for the new independent branch test.

Downstream context

The dispat acceptance record covers files, environment variables, pipes, child processes, goroutines, network clients, real TLS, certificate rejection and self-update rollback. See the recorded acceptance. Those results are for the combined fork and the revisions named in that document. They do not prove that this one PR supplies all those features.

Related pull requests

The target partition below keeps each change in one PR. The process branch cleanup is still in progress.

• sync: hand the RWMutex over instead of a re-test of the reader count #5630 fixes RWMutex handover.
• runtime: deliver os/signal notifications under the threads scheduler #5631 is closed in favor of runtime: deliver signals under the threads scheduler when blocked on I/O #5530, owned by @0pcom. The fork still uses the tested runtime: deliver os/signal notifications under the threads scheduler #5631 implementation until a replacement is checked.
• compiler,runtime: call fcntl through a C wrapper on darwin #5632 is closed in favor of compiler: implement go:cgo_import_dynamic and lower Darwin cgo import trampolines (#5604) #5612, owned by @neomantra. compiler: implement go:cgo_import_dynamic and lower Darwin cgo import trampolines (#5604) #5612 covers both stdlib and x/sys Darwin trampolines.
• runtime: implement weak.runtime_makeStrongFromWeak #5633 provides the weak-pointer runtime hook.
• os: start processes with posix_spawn #5634 provides process spawning and process state. Darwin also needs compiler: implement go:cgo_import_dynamic and lower Darwin cgo import trampolines (#5604) #5612 and builder: declare Darwin socket and process symbols #5636. Concurrent use needs sync: hand the RWMutex over instead of a re-test of the reader count #5630.
• loader: use the real crypto/tls on hosted linux and darwin #5635 selects software TLS on hosted Linux and Darwin. It needs runtime: implement weak.runtime_makeStrongFromWeak #5633. The overlapping crypto/tls: use Go's software TLS on the host target #5645 is under discussion with @0pcom.
• builder: declare Darwin socket and process symbols #5636 provides the Darwin socket and process link symbols. It does not include process or TLS implementation changes.

In tinygo-org/net

• net/http: follow redirects in the client again net#72 restores HTTP redirects. It shares http/client.go with the timeout work in http: port Go net/http timeout handling net#67, so the two changes need integration review.
• net: report a name that does not exist as a not-found lookup error net#73 preserves DNS not-found errors.
• net: extend the host netdev to darwin net#74 adds Darwin host netdev support. It shares the host netdev with net: non-blocking host netdev I/O with an epoll poller net#77. Linux epoll and Darwin support must remain separate.
• net/http: give the HTTPS client trust roots on darwin net#75 supplies Darwin trust roots to the HTTP client.
• net: non-blocking host netdev I/O with an epoll poller net#77 owns Linux cancellation and deadline work, net: report the kernel-assigned port after listening on port 0 net#80 owns port-zero reporting, and net/http: add ListenAndServeTLS / ServeTLS (software TLS) net#79 owns HTTP server TLS. These are separate contributors' changes, not completed parts of this series.
• net: API-compat fills for grpc and wazero (SyscallConn, Dialer.Control, IPConn.Read) net#78 adds client API fields and related compatibility methods.

Full Darwin networking also needs the merged net changes and a later src/net pin update. No upstream merge or current full-suite pass is implied by this list.

Additional independent gaps found in Crier are covered by #5655 (cookie-jar loader merge) and tinygo-org/net#82 (ListenConfig). They do not duplicate the process, TLS, deadline or server-TLS changes above. Current Crier comparison work is separate from its standard-Go release.

Current Crier comparison

The same-source Crier report records 143 passing Linux ARM64 end-to-end tests for Go, TinyGo, and a symbol-stripped TinyGo copy. The last is 13,829,248 bytes versus Go's 30,277,794 bytes. The tests cover the recorded CLI paths, not general Go compatibility. Two render fixtures exceed pixel tolerance, and the tested Darwin TinyGo binary fails before main. This evidence uses the earlier released compiler with the exact source overlays named in the report. It does not validate this PR head or the new fork candidate. The standard-Go Crier release is separate.

[yohimik]

Rebased on dev after the 0.42.0 release. The change applies on top of v0.42.0
as released without a conflict, and the diff is unchanged.