cgo: support for mingw-w64 4.5.1 and newer

R=rsc, jp, hectorchu
CC=golang-dev
https://golang.org/cl/4962051

src/cmd/ld/ldpe.c

@@ -408,13 +408,15 @@ readsym(PeObj *obj, int i, PeSym **y)
 	sym = &obj->pesym[i];
 	*y = sym;
 	
-	name = sym->name;
-	if(sym->sclass == IMAGE_SYM_CLASS_STATIC && sym->value == 0) // section
+	if(sym->sclass == IMAGE_SYM_CLASS_STATIC && sym->value == 0 && sym->type == 0) // section
 		name = obj->sect[sym->sectnum-1].sym->name;
-	if(strncmp(sym->name, "__imp__", 7) == 0)
-		name = &sym->name[7]; // __imp__Name => Name
-	else if(sym->name[0] == '_') 
-		name = &sym->name[1]; // _Name => Name
+	else {
+		name = sym->name;
+		if(strncmp(name, "__imp_", 6) == 0)
+			name = &name[6]; // __imp_Name => Name
+		if(thechar == '8' && name[0] == '_')
+			name = &name[1]; // _Name => Name
+	}
 	// remove last @XXX
 	p = strchr(name, '@');
 	if(p)
@@ -443,8 +445,8 @@ 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__", 7) == 0)
-		s->got = -2; // flag for __imp__
+	if(strncmp(sym->name, "__imp_", 6) == 0)
+		s->got = -2; // flag for __imp_
 	sym->sym = s;
 
 	return 0;

src/pkg/runtime/cgo/amd64.S

@@ -5,7 +5,7 @@
 /*
  * Apple still insists on underscore prefixes for C function names.
  */
-#if defined(__APPLE__) || defined(_WIN32)
+#if defined(__APPLE__)
 #define EXT(s) _##s
 #else
 #define EXT(s) s