Mostly NOSPLIT additions.
Had to rewrite atomic_arm.c in Go because it calls lock,
and lock is too complex.

With this CL, I find no Go -> C calls that can split the stack
on any system except Solaris and Windows.

Solaris and Windows need more work and will be done separately.

LGTM=iant, dave
R=golang-codereviews, bradfitz, iant, dave
CC=dvyukov, golang-codereviews, khr, r
https://golang.org/cl/137160043