bsd-user: Create setup_sigframe_arch to setup sigframe context

Define setup_sigframe_arch whose job it is to setup the mcontext for the
sigframe. Implement for x86 to just call mcontext.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/bsd-user/freebsd/target_os_signal.h b/bsd-user/freebsd/target_os_signal.h
index 3ed454e..43700d0 100644
--- a/bsd-user/freebsd/target_os_signal.h
+++ b/bsd-user/freebsd/target_os_signal.h
@@ -4,6 +4,9 @@
 #include "target_os_siginfo.h"
 #include "target_arch_signal.h"
 
+abi_long setup_sigframe_arch(CPUArchState *env, abi_ulong frame_addr,
+                             struct target_sigframe *frame, int flags);
+
 /* Compare to sys/signal.h */
 #define TARGET_SIGHUP  1       /* hangup */
 #define TARGET_SIGINT  2       /* interrupt */