cmd/ld: refine test for history stack pop (fix Windows build)
cmd/ld: refine test for history stack pop (fix Windows build)

This should fix the Windows build, or at least
what's breaking it at the moment.

Fixes issue 5904.

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/11519044

apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme