cmd/ld: attempt at fixing openbsd build

OpenBSD is excluded from all the usual thread-local storage
code, not just emitting the tbss section in the external link .o
but emitting a PT_TLS section in an internally-linked executable.
I assume it just has no proper TLS support. Exclude it here too.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/87900045

src/cmd/ld/data.c

@@ -184,7 +184,7 @@ relocsym(LSym *s)
 				o = r->add;
 			break;
 		case R_TLS_LE:
-			if(linkmode == LinkExternal && iself) {
+			if(linkmode == LinkExternal && iself && HEADTYPE != Hopenbsd) {
 				r->done = 0;
 				r->sym = ctxt->gmsym;
 				r->xsym = ctxt->gmsym;
@@ -198,7 +198,7 @@ relocsym(LSym *s)
 			break;
 
 		case R_TLS_IE:
-			if(linkmode == LinkExternal && iself) {
+			if(linkmode == LinkExternal && iself && HEADTYPE != Hopenbsd) {
 				r->done = 0;
 				r->sym = ctxt->gmsym;
 				r->xsym = ctxt->gmsym;