HAVE_RESTORE_SIGMASK is defined on all architectures now

Everyone either defines it in arch thread_info.h or has TIF_RESTORE_SIGMASK
and picks default set_restore_sigmask() in linux/thread_info.h.  Kill the
ifdefs, slap #error in linux/thread_info.h to catch breakage when new ones
get merged.

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

include/linux/thread_info.h

@@ -129,6 +129,10 @@ static inline void set_restore_sigmask(void)
 }
 #endif	/* TIF_RESTORE_SIGMASK && !HAVE_SET_RESTORE_SIGMASK */
 
+#ifndef HAVE_SET_RESTORE_SIGMASK
+#error "no set_restore_sigmask() provided and default one won't work"
+#endif
+
 #endif	/* __KERNEL__ */
 
 #endif /* _LINUX_THREAD_INFO_H */