runtime: convert channel operations to Go, part 1 (chansend1).

LGTM=dvyukov
R=dvyukov, khr
CC=golang-codereviews
https://codereview.appspot.com/127460044

Committer: Dmitriy Vyukov <dvyukov@google.com>
runtime: convert synchronous semaphores to Go

LGTM=rsc
R=golang-codereviews, khr, rsc
CC=golang-codereviews, rlh
https://codereview.appspot.com/130340043