cmd/ld: fix 'use after free' error in new obj code

Many thanks to Elias Naur for finding this with Valgrind on Linux.
Perhaps this is what is breaking the windows/amd64 builder.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7595044

src/cmd/ld/lib.c

@@ -528,7 +528,7 @@ ldhostobj(void (*ld)(Biobuf*, char*, int64, char*), Biobuf *f, char *pkg, int64
 	}
 	h = &hostobj[nhostobj++];
 	h->ld = ld;
-	h->pkg = pkg;
+	h->pkg = estrdup(pkg);
 	h->pn = estrdup(pn);
 	h->file = estrdup(file);
 	h->off = Boffset(f);