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://codereview.appspot.com/136330043