cpus.c: Use pthread_sigmask() rather than sigprocmask()

On Linux, sigprocmask() and pthread_sigmask() are in practice the
same thing (they only set the signal mask for the calling thread),
but the documentation states that the behaviour of sigprocmask() in a
multithreaded process is undefined. Use pthread_sigmask() instead
(which is what we do in almost all places in QEMU that alter the
signal mask already).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1463420039-29761-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -780,7 +780,7 @@ static void sigbus_reraise(void)
         raise(SIGBUS);
         sigemptyset(&set);
         sigaddset(&set, SIGBUS);
-        sigprocmask(SIG_UNBLOCK, &set, NULL);
+        pthread_sigmask(SIG_UNBLOCK, &set, NULL);
     }
     perror("Failed to re-raise SIGBUS!\n");
     abort();