macOS: enable guest signal dispatch on the target thread (#136)

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.

src/common/hostException.cpp

@@ -324,6 +324,10 @@ bool InstallHandler(Handler handler) {
 	sa.sa_sigaction = SignalHandler;
 	sa.sa_flags     = SA_SIGINFO;
 	sigemptyset(&sa.sa_mask);
+	// The guest signal-dispatch path (KernelRaiseException) interrupts threads with
+	// SIGUSR1; block it while a fault is being resolved so a stop-the-world request
+	// cannot preempt the handler between the protection fix and the retry.
+	sigaddset(&sa.sa_mask, SIGUSR1);
 
 	// macOS raises SIGBUS for protection faults on some paths and SIGSEGV on others;
 	// SIGILL covers instructions the host cannot execute (routed to the x64 emulator).