[runtime] Fix -Wcast-function-type-mismatch warnings; NFC (3/4)

__abort_sig_hand() resets signal handlers before printing an error message
and calling abort(). The code used to set the sa_sigaction field to SIG_DFL,
requiring an explicit cast which now causes a warning. It is actually not
necessary to use sa_sigaction in this case; setting sa_handler to SIG_DFL
is the idiomatic way which also avoids the warning.

runtime/flangrti/trace_lin.c

@@ -215,9 +215,8 @@ __abort_sig_hand(int sig, siginfo_t *in, FLANGRTI_UCONTEXT_T *u)
   struct sigaction new;
   struct sigaction old;
 
-  new.sa_sigaction = (void (*)(int, siginfo_t *, void *))SIG_DFL;
+  new.sa_handler = SIG_DFL;
   sigemptyset(&new.sa_mask);
-  new.sa_flags = SA_SIGINFO;
   n = 0;
   while (sigs[n].sig != 0) {
     sigaction(sigs[n].sig, &new, &old);