Convert no-op race functions.
Everything else is tiny and gets NOSPLITs.

After this, all that is left on darwin is sysAlloc, panic, and gothrow (all pending).
There may be system-specific calls in other builds.

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, khr, r
https://golang.org/cl/140240044