Replace CoreCLR PAL wait subsystem

[jkoritzinsky]

CoreCLR's remaining Unix native waits are single-handle event waits, so the legacy PAL synchronization manager and its waitable handle infrastructure are no longer needed.

This change:

• moves the NativeAOT event implementation into src/coreclr/runtime for shared CoreCLR and NativeAOT use, with direct Windows forwarding and a pthread-based Unix implementation;
• adds PAL_Sleep, backed by Sleep on Windows and EINTR-safe nanosleep on Unix;
• migrates Unix-reachable CoreCLR event and sleep callers to the shared PAL;
• adds a Unix-only manual-reset thread-exit event so CoreCLR waits on that event instead of a PAL thread handle, while Windows continues waiting on the OS thread handle; and
• removes the old PAL event, sleep, thread-wait, synchronization manager, controller, cache, and associated test infrastructure.

Testing

• ./build.sh clr+libs+host
• ./build.sh clr.native+clr.paltests
• PAL test suite: 256 passed, 0 failed

Note

This pull request description was created by GitHub Copilot.

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