runtime: fix semacquire->acquireSudog->malloc->gogc->semacquire loop

This is what broke the build at
http://build.golang.org/log/d9c6d334be16cbab85e99fddc6b4ba034319bd4e

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, khr, r
https://codereview.appspot.com/135580043