runtime: fix gogetenv on Plan 9
runtime: fix gogetenv on Plan 9

LGTM=rsc
R=rsc, ality
CC=golang-codereviews
https://codereview.appspot.com/137030043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go