cmd/ld: maintain correct headstring needed by windows port.

Fixes build for go_bootstrap.
Fixes #6971.

R=alex.brainman, mattn.jp, rsc
CC=golang-dev
https://golang.org/cl/43130043

src/cmd/ld/lib.c

@@ -1255,6 +1255,7 @@ setheadtype(char *s)
 		fprint(2, "unknown header type -H %s\n", s);
 		errorexit();
 	}
+	headstring = s;
 	HEADTYPE = headtype(s);
 }
 

src/cmd/ld/pobj.c

@@ -138,6 +138,8 @@ main(int argc, char *argv[])
 	if(HEADTYPE == -1)
 		HEADTYPE = headtype(goos);
 	ctxt->headtype = HEADTYPE;
+	if (headstring == NULL)
+		headstring = headstr(HEADTYPE);
 
 	archinit();
 	ctxt->linkmode = linkmode;