macos: enable guest signal dispatch on the target thread
The POSIX signal-dispatch path (pthread_kill based, added with the Linux
port) was compiled out on macOS, leaving KernelRaiseException to run the
guest handler on the calling thread. IL2CPP's garbage collector raises its
stop-the-world signal at every managed thread and each handler parks its
own thread until resume, so the collector parked itself and every Unity
title froze on the first collection.
Enable the same delivery path on macOS:
- translate between the Darwin mcontext (uc_mcontext->__ss) and the guest
ucontext in CreateSignalUcontextFromHost/ApplySignalUcontextToHost
- use SIGUSR1 as the host dispatch signal (macOS has no realtime signals)
- block the dispatch signal inside the host fault handler so a suspend
request cannot preempt fault resolution between the protection fix and
the retry
Windows and Linux are unchanged.
Chai T