runtime: fix stack cleanup on windows/amd64
runtime: fix stack cleanup on windows/amd64

In amd64 it is the caller who cleans the stack.

R=alex.brainman, vcc.163, jp
CC=golang-dev
http://codereview.appspot.com/4966046

Committer: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go