cmd/cc: fix undefined behaviour warning in bv.c

Fixes warning

# _/home/dfc/go/misc/cgo/test/backdoor
/home/dfc/go/src/cmd/cc/bv.c:43:11: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/136330043

src/cmd/cc/bv.c

@@ -40,6 +40,6 @@ bvset(Bvec *bv, int32 i)
 
 	if(i < 0 || i >= bv->n)
 		fatal(Z, "bvset: index %d is out of bounds with length %d\n", i, bv->n);
-	mask = 1 << (i % WORDBITS);
+	mask = 1UL << (i % WORDBITS);
 	bv->b[i / WORDBITS] |= mask;
 }