[NativeAOT] Use POSIX primitives for GC bridge processing

[simonrozsival]

Summary

Replace the NativeAOT-reachable GC bridge's C++ threading primitives with POSIX primitives.

This removes the std::thread, std::binary_semaphore, and associated libc++ atomic-wait/thread runtime dependency from this subsystem as part of #12139.

Changes

• replace the detached std::thread worker with pthread_create() / pthread_detach();
• replace std::binary_semaphore with sem_t;
• retry sem_wait() after EINTR;
• preserve acquire/release synchronization for the shared callback argument with compiler atomics;
• surface pthread/semaphore failures through the existing abort helpers.

The bridge-processing algorithm and callback lifecycle are unchanged.

Validation

• git diff --check
• Release build of src/native/native-nativeaot.csproj
• Release build of src/native/native-clr.csproj
• focused concurrency review covering initialization, memory ordering, EINTR, and detached worker behavior

Part of #12139.

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