avoid the thread-ptr-init behavior of sigaction when not installing handler

this is necessary because posix_spawn calls sigaction after vfork, and
if the thread pointer is not already initialized, initializing it in
the child corrupts the parent process's state.

src/signal/sigaction.c

@@ -15,12 +15,13 @@ int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigact
 {
 	struct k_sigaction ksa;
 	if (sa) {
+		if ((uintptr_t)sa->sa_handler > 1UL)
+			__pthread_self_def();
 		ksa.handler = sa->sa_handler;
 		ksa.flags = sa->sa_flags | SA_RESTORER;
 		ksa.restorer = (sa->sa_flags & SA_SIGINFO) ? __restore_rt : __restore;
 		memcpy(&ksa.mask, &sa->sa_mask, sizeof ksa.mask);
 	}
-	__pthread_self_def();
 	if (syscall(SYS_rt_sigaction, sig, sa?&ksa:0, old?&ksa:0, sizeof ksa.mask))
 		return -1;
 	if (old) {