cmd/gc: fix warning on Plan 9

warning: src/cmd/gc/bits.c:101 non-interruptable temporary

Change-Id: I74661fefab50455b912b8085d913fc45ba13c5c8
Reviewed-on: https://go-review.googlesource.com/1780Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/gc/bits.c

@@ -97,9 +97,11 @@ bnum(Bits a)
 	int i;
 	uint64 b;
 
-	for(i=0; i<BITS; i++)
-		if(b = a.b[i])
+	for(i=0; i<BITS; i++){
+		b = a.b[i];
+		if(b)
 			return 64*i + bitno(b);
+	}
 	fatal("bad in bnum");
 	return 0;
 }