consolidate declarations of k_sigaction

Only alpha and sparc are unusual - they have ka_restorer in it.
And nobody needs that exposed to userland.

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

arch/alpha/include/asm/signal.h

@@ -28,9 +28,6 @@ struct sigaction {
 	sigset_t	sa_mask;	/* mask last for extensibility */
 };
 
-struct k_sigaction {
-	struct sigaction sa;
-	__sigrestore_t ka_restorer;
-};
+#define __ARCH_HAS_KA_RESTORER
 #include <asm/sigcontext.h>
 #endif