Add OpenHarmony (HarmonyOS) support to the .NET runtime

[springmin]

Summary

.NET currently supports Android (linux-bionic) and the Apple mobile platforms (ios-*, tvos-*, maccatalyst-*), but not OpenHarmony / HarmonyOS. OpenHarmony is an open-source operating system for phones, tablets, and IoT devices, and HarmonyOS NEXT is the commercial distribution built on it. Adding .NET support would allow C#/.NET applications and libraries to run on this platform.

This issue tracks the overall porting effort. The work is split into a series of PRs so each piece can be reviewed independently.

Scope

1. Runtime sandbox fixes — OpenHarmony app sandboxes differ from a plain Linux environment in ways that break .NET at startup or at runtime:
   • /tmp is mounted read-only; shared-memory files must honor TMPDIR.
   • The get_mempolicy/mbind syscalls are blocked by the sandbox's seccomp policy, so the GC NUMA probe SIGSYS-crashes the process at startup.
   • The sysroot's pthread lacks robust-mutex support, so NamedMutex must use the shared-memory-file implementation.
   • Tracked by: Handle read-only /tmp and missing NUMA/robust-mutex on HarmonyOS #132827 (first PR of the series, currently ready for review).
2. Build infrastructure — -os linux-ohos support: RID graph entry, NDK/sysroot toolchain plumbing, and CI cross-build legs mirroring linux-bionic.
3. Runtime (CoreCLR/Mono) enablement — platform-level bring-up, PAL adjustments, and validation on OpenHarmony devices/emulators.
4. Tests & validation — targeted test legs for the linux-ohos RID.

Approach

• Follow the existing linux-bionic (Android) and linux-musl patterns for cross-compilation and RID plumbing.
• Keep every PR independently reviewable and a no-op on existing platforms: new behavior is guarded by TARGET_OHOS, which is only defined when the new target is selected.

Open questions

• Which HarmonyOS API level and NDK version to target.
• Whether to support both OpenHarmony (open source) and HarmonyOS NEXT (commercial) from the start.
• Mono vs CoreCLR prioritization for the first milestone.

Note

This issue was authored with AI assistance (Copilot/agent tooling) under the repository owner's direction.

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[huoyaoyuan]

Already tracked at #103627.