cmd/go: enforce -Wl,--push-state logic only when it is a prefix

cmd/go: enforce -Wl,--push-state logic only when it is a prefix

Make sure we only run the --push-state logic when -Wl,--push-state is a
prefix of the argument, not just present in the argument string.

Thanks to Juho Forsén of Mattermost for reporting this issue.

Change-Id: I799f7854ff680674fd84cf2136fadf70817fc7e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/628415
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>