exec.c: Initialize sa_flags passed to sigaction()

Coverity points out that in the user-only version of cpu_abort() we
call sigaction() with a partially initialized struct sigaction
(CID 1005351). Correct the omission.

Backports commit 8347c18506c3f8619527d19134cb4aac071dc54a from qemu

qemu/exec.c

@@ -939,6 +939,7 @@ void cpu_abort(CPUState *cpu, const char *fmt, ...)
         struct sigaction act;
         sigfillset(&act.sa_mask);
         act.sa_handler = SIG_DFL;
+        act.sa_flags = 0;
         sigaction(SIGABRT, &act, NULL);
     }
 #endif