switch compat_sys_sigaltstack() to COMPAT_SYSCALL_DEFINE

Makes sigaltstack conversion easier to split into per-architecture
parts.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/signal.c

@@ -3119,8 +3119,9 @@ int __save_altstack(stack_t __user *uss, unsigned long sp)
 
 #ifdef CONFIG_COMPAT
 #ifdef CONFIG_GENERIC_SIGALTSTACK
-asmlinkage long compat_sys_sigaltstack(const compat_stack_t __user *uss_ptr,
-				       compat_stack_t __user *uoss_ptr)
+COMPAT_SYSCALL_DEFINE2(sigaltstack,
+			const compat_stack_t __user *, uss_ptr,
+			compat_stack_t __user *, uoss_ptr)
 {
 	stack_t uss, uoss;
 	int ret;