Improve crash handler on XNU

This avoids an issue where a crash signal could cause the MacOS process
to freeze and consume all CPU rather than dying as it rightfully should

test/libc/calls/signal_test.c

@@ -53,7 +53,7 @@ void OnSigTrap(int sig, struct siginfo *si, void *ctx) {
 
 void TrapBench(int n) {
   for (int i = 0; i < n; ++i) {
-    __builtin_trap();
+    DebugBreak();
   }
 }