runtime: make more functions safe for Go

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

src/pkg/runtime/os_openbsd.c

@@ -164,6 +164,7 @@ runtime·osinit(void)
 	runtime·ncpu = getncpu();
 }
 
+#pragma textflag NOSPLIT
 void
 runtime·get_random_data(byte **rnd, int32 *rnd_len)
 {