runtime/cgo: block signals to the iOS mach exception handler

For darwin/arm{,64} a non-Go thread is created to convert
EXC_BAD_ACCESS to panics. However, the Go signal handler refuse to
handle signals that would otherwise be ignored if they arrive at
non-Go threads.

Block all (posix) signals to that thread, making sure that
no unexpected signals arrive to it. At least one test, TestStop in
os/signal, depends on signals not arriving on any non-Go threads.

For #14318

Change-Id: I901467fb53bdadb0d03b0f1a537116c7f4754423
Reviewed-on: https://go-review.googlesource.com/21047Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/cgo/gcc_signal_darwin_armx.c

@@ -182,6 +182,7 @@ darwin_arm_init_mach_exception_handler()
 	int ret;
 	pthread_t thr = NULL;
 	pthread_attr_t attr;
+	sigset_t ign, oset;
 
 	ret = mach_port_allocate(
 		mach_task_self(),
@@ -192,11 +193,18 @@ darwin_arm_init_mach_exception_handler()
 		abort();
 	}
 
+	// Block all signals to the exception handler thread
+	sigfillset(&ign);
+	pthread_sigmask(SIG_SETMASK, &ign, &oset);
+
 	// Start a thread to handle exceptions.
 	uintptr_t port_set = (uintptr_t)mach_exception_handler_port_set;
 	pthread_attr_init(&attr);
 	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
 	ret = pthread_create(&thr, &attr, mach_exception_handler, (void*)port_set);
+
+	pthread_sigmask(SIG_SETMASK, &oset, nil);
+
 	if (ret) {
 		fprintf(stderr, "runtime/cgo: pthread_create failed: %d\n", ret);
 		abort();