cmd/ld: fix OpenBSD (third try)

On OpenBSD don't mark runtime.{g,m} as STT_TLS symbols.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/7867046

src/cmd/ld/symtab.c

@@ -153,7 +153,7 @@ asmelfsym(void)
 	elfbind = STB_LOCAL;
 	genasmsym(putelfsym);
 	
-	if(linkmode == LinkExternal) {
+	if(linkmode == LinkExternal && HEADTYPE != Hopenbsd) {
 		s = lookup("runtime.m", 0);
 		if(s->sect == nil) {
 			cursym = nil;