runtime: implement race-free signals using futexes

This requires an API introduced in MacOS 11. I think that's fine, since
the version before that (MacOS 10.15) is EOL since 2022. Though if
needed, we could certainly work around it by using an older and slightly
less nice API.

src/runtime/signal.c

@@ -30,61 +30,3 @@ void tinygo_signal_disable(uint32_t sig) {
     act.sa_handler = SIG_DFL;
     sigaction(sig, &act, NULL);
 }
-
-// Implement waitForEvents and sleep with signals.
-// Warning: sigprocmask is not defined in a multithreaded program so will need
-// to be replaced with something else once we implement threading on POSIX.
-
-// Signals active before a call to tinygo_wfi_mask.
-static sigset_t active_signals;
-
-static void tinygo_set_signals(sigset_t *mask, uint32_t signals) {
-    sigemptyset(mask);
-    for (int i=0; i<32; i++) {
-        if ((signals & (1<<i)) != 0) {
-            sigaddset(mask, i);
-        }
-    }
-}
-
-// Mask the given signals.
-// This function must always restore the previous signals using
-// tinygo_wfi_unmask, to create a critical section.
-void tinygo_wfi_mask(uint32_t active) {
-    sigset_t mask;
-    tinygo_set_signals(&mask, active);
-
-    sigprocmask(SIG_BLOCK, &mask, &active_signals);
-}
-
-// Wait until a signal becomes pending (or is already pending), and return the
-// signal.
-#if !defined(__APPLE__)
-int tinygo_wfi_sleep(uint32_t active, uint64_t timeout) {
-    sigset_t active_set;
-    tinygo_set_signals(&active_set, active);
-
-    struct timespec ts = {0};
-    ts.tv_sec  = timeout / 1000000000;
-    ts.tv_nsec = timeout % 1000000000;
-
-    int result = sigtimedwait(&active_set, NULL, &ts);
-    return result;
-}
-#endif
-
-// Wait until any of the active signals becomes pending (or returns immediately
-// if one is already pending).
-int tinygo_wfi_wait(uint32_t active) {
-    sigset_t active_set;
-    tinygo_set_signals(&active_set, active);
-
-    int sig = 0;
-    sigwait(&active_set, &sig);
-    return sig;
-}
-
-// Restore previous signal mask.
-void tinygo_wfi_unmask(void) {
-    sigprocmask(SIG_SETMASK, &active_signals, NULL);
-}