coroutine-sigaltstack.c: Use stack_t, not struct sigaltstack

Use the POSIX-specified stack_t type as the argument to sigaltstack()
rather than the legacy struct sigaltstack. This allows us to compile
on MacOSX with --with-coroutine=sigaltstack.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

coroutine-sigaltstack.c

@@ -171,8 +171,8 @@ static Coroutine *coroutine_new(void)
     CoroutineThreadState *coTS;
     struct sigaction sa;
     struct sigaction osa;
-    struct sigaltstack ss;
-    struct sigaltstack oss;
+    stack_t ss;
+    stack_t oss;
     sigset_t sigs;
     sigset_t osigs;
     jmp_buf old_env;