Protect bzero call against macro expansion

Shortly, we'll be moving to defining bzero and memset in terms of
__builting_memset. To do that, we can't have macro calls to bzero in
the fallback impelmentation of memset. Normal calls to bzero are fine.
All 4 architectures that use this have their own copies of bzero, so
there's no mutual recursion issue between memset and bcopy.

sys/libkern/memset.c

@@ -38,7 +38,7 @@ memset(void *b, int c, size_t len)
 	char *bb;
 
 	if (c == 0)
-		bzero(b, len);
+		(bzero)(b, len);
 	else
 		for (bb = (char *)b; len--; )
 			*bb++ = c;