Ignore sigfillset

Follow a suggestion by Christiano Haesbaert, as Solo5 doesn't provide
signals, it might just be ignored
As of OCaml 5.2.0 beta1, sigfillset is only used in functions creating
threads, and so that will abort in any case. Ignoring the function may
make it more future-proof, though.

nolibc/stubs.c

@@ -148,7 +148,7 @@ STUB_IGNORE(int, pthread_cond_broadcast, 0);
 STUB_ABORT(pthread_self);
 STUB_ABORT(pthread_detach);
 
-STUB_ABORT(sigfillset);
+STUB_IGNORE(sigfillset);
 STUB_ABORT(sigwait);
 STUB_ABORT(usleep);
 STUB_ABORT(strerror_r);