cmd/gc: fix undefined behaviour warning in subr.c

Fixes warning

/home/dfc/go/src/cmd/gc/subr.c:3469:8: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64' (aka 'long'); cast to an unsigned type to negate this value to itself

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/141220043