[release-branch.go1.4] cmd/go: fix running pprof on windows.

««« CL 176170043 / 61bbf19823d5
cmd/go: fix running pprof on windows.

Fixes issue 9149.

LGTM=alex.brainman, rsc
R=rsc, dave, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/176170043

Committer: Russ Cox <rsc@golang.org>
»»»

TBR=minux
CC=golang-codereviews
https://codereview.appspot.com/175550043