Fix concurrent Unix named mutex creation

[jkoritzinsky]

Why

Concurrent shared compilations on Linux can intermittently fail when a newly created named mutex is initially owned. The creator can lose the mutex between publishing the shared state and recording ownership, causing a later ReleaseMutex to fail with "Cannot release a lock that is not owned by the current thread."

What changed

• Keep the creation/deletion process lock held through nonblocking initial mutex acquisition and ownership bookkeeping.
• Preserve the existing behavior of releasing that lock before blocking waits.
• Add a Unix regression test covering concurrent same-name constructors with initial ownership.

Testing

• ./build.sh clr+libs -rc checked
• ./build.sh clr.corelib+clr.nativecorelib+libs.pretest -rc checked
• dotnet test src/libraries/System.Threading/tests/System.Threading.Tests.csproj --no-build (714 passed)
• Focused regression test passed.

Resolves #134043

Note

This pull request description was generated with GitHub Copilot assistance.

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Copilot review overview

🟡 Changes recommended

Add platform capability guards to prevent unsupported targets from failing or hanging.

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Review effort: Lite
Findings: 1

Open (1)

• Skip platforms without Unix named-mutex multithreading support · New

What changed in this PR

Fixes a Unix named-mutex creation race by preserving the process lock through initial acquisition and ownership bookkeeping, with a concurrent regression test.

Changes:

• Updates Unix mutex acquisition locking.
• Adds concurrent initial-ownership coverage.
• Test requires platform capability guards for unsupported targets.

File	Description
src/​libraries/​System.Threading/​tests/​MutexTests.cs	Adds concurrent named-mutex constructor coverage; requires guards for mobile, Browser, and Wasi.
src/​libraries/​System.Private.CoreLib/​src/​System/​Threading/​NamedMutex.Unix.cs	Preserves locking during nonblocking ownership setup.