runtime: more C to Go conversion adjustments

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

src/cmd/api/goapi.go

@@ -411,6 +411,7 @@ func (w *Walker) parseFile(dir, file string) (*ast.File, error) {
 			"); " +
 			"const (" +
 			" cb_max = 2000;" +
+			" _CacheLineSize = 64;" +
 			" _Gidle = 1;" +
 			" _Grunnable = 2;" +
 			" _Grunning = 3;" +