cmd/gc: avoid %#x of 0

Plan 9 and Go's lib9/fmt disagree on whether %#x includes the 0x prefix
when printing 0, because ANSI C gave bad advice long ago.

Avoiding that case makes binaries compiled on different systems compatible.

R=ken2
CC=akumar, golang-dev
https://golang.org/cl/6814066

src/cmd/gc/esc.c

@@ -233,7 +233,7 @@ mktag(int mask)
 	if(mask < nelem(tags) && tags[mask] != nil)
 		return tags[mask];
 
-	snprint(buf, sizeof buf, "esc:%#x", mask);
+	snprint(buf, sizeof buf, "esc:0x%x", mask);
 	s = strlit(buf);
 	if(mask < nelem(tags))
 		tags[mask] = s;