remove implementation-reserved bits when saving signal mask

this fix is necessary because a program could be started with some of
the implementation-reserved signals masked (e.g. due to exec having
been called from a signal handler, or from a non-musl program) and
then could obtain an invalid-to-use-later sigset_t as the old/saved
signal mask.

src/thread/pthread_sigmask.c

@@ -5,6 +5,16 @@
 
 int pthread_sigmask(int how, const sigset_t *set, sigset_t *old)
 {
+	int ret;
 	if (how > 2U) return EINVAL;
-	return -__syscall(SYS_rt_sigprocmask, how, set, old, 8);
+	ret = -__syscall(SYS_rt_sigprocmask, how, set, old, 8);
+	if (!ret && old) {
+		if (sizeof old->__bits[0] == 8) {
+			old->__bits[0] &= ~0x380000000ULL;
+		} else {
+			old->__bits[0] &= ~0x80000000UL;
+			old->__bits[1] &= ~0x3UL;
+		}
+	}
+	return ret;
 }