bsd-user/signal.c: setup_frame

setup_frame sets up a signalled stack frame. Associated routines to
extract the pointer to the stack frame and to support alternate stacks.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Kyle Evans <kevans@freebsd.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

bsd-user/main.c

@@ -217,6 +217,11 @@ void qemu_cpu_kick(CPUState *cpu)
 /* Assumes contents are already zeroed.  */
 static void init_task_state(TaskState *ts)
 {
+    ts->sigaltstack_used = (struct target_sigaltstack) {
+        .ss_sp = 0,
+        .ss_size = 0,
+        .ss_flags = TARGET_SS_DISABLE,
+    };
 }
 
 void gemu_log(const char *fmt, ...)