syscalls.master: align with sigfastblock declaration

sigfastblock is declared to take a void * argument in the manpage in
headers so declare it that way and use SAL annotations to say it
interacts with a 32-bit word.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D44379

sys/kern/syscalls.master

@@ -3231,7 +3231,7 @@
 573	AUE_NULL	STD|CAPENABLED {
 		int sigfastblock(
 		    int cmd,
-		    _Inout_opt_ uint32_t *ptr
+		    _Inout_updates_bytes_opt_(4) void *ptr
 		);
 	}
 574	AUE_REALPATHAT	STD {