ld: fix link Windows PE __declspec(dllimport) symbol

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4568077

src/cmd/ld/data.c

@@ -249,7 +249,7 @@ dynrelocsym(Sym *s)
 			return;
 		for(r=s->r; r<s->r+s->nr; r++) {
 			targ = r->sym;
-			if(r->sym->plt == -2) { // make dynimport JMP table for PE object files.
+			if(r->sym->plt == -2 && r->sym->got != -2) { // make dynimport JMP table for PE object files.
 				targ->plt = rel->size;
 				r->sym = rel;
 				r->add = targ->plt;

src/cmd/ld/ldpe.c

@@ -403,6 +403,8 @@ readsym(PeObj *obj, int i, PeSym **y)
 
 	if(s != nil && s->type == 0 && !(sym->sclass == IMAGE_SYM_CLASS_STATIC && sym->value == 0))
 		s->type = SXREF;
+	if(strncmp(sym->name, "__imp__", 6) == 0)
+		s->got = -2; // flag for __imp__
 	sym->sym = s;
 
 	return 0;