runtime: use SA_RESTART when registering a signal

This really is the only sane way to register a signal. If this flag is
not set, many syscalls will return EINTR (and not complete their
operation) which will be a massive source of hard-to-debug bugs.

src/runtime/signal.c

@@ -15,6 +15,7 @@ void tinygo_signal_handler(int sig);
 void tinygo_signal_enable(uint32_t sig) {
     struct sigaction act = { 0 };
     act.sa_handler = &tinygo_signal_handler;
+    act.sa_flags = SA_RESTART;
     sigaction(sig, &act, NULL);
 }