cmd/link: do not add __image_base__ and _image_base__ if external linker

The symbols get in a way when using external linker. They are
not associated with a section. And linker fails when
generating relocations for them.

__image_base__ and _image_base__ have been added long time ago.
I do not think they are needed anymore. If I delete them, all
tests still PASS. I tried going back to the commit that added
them to see if I can reproduce original error, but I cannot
build it. I don't have hg version of go repo, and my gcc is
complaining about cc source code.

I wasted too much time with this, so I decided to leave them only
for internal linker. That is what they were originally added for.

For #10776.

Change-Id: Ibb72b04f3864947c782f964a7badc69f4b074e25
Reviewed-on: https://go-review.googlesource.com/36979Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/pe.go

@@ -458,10 +458,11 @@ func Peinit(ctxt *Link) {
 	nextsectoff = int(PESECTHEADR)
 	nextfileoff = int(PEFILEHEADR)
 
-	// some mingw libs depend on this symbol, for example, FindPESectionByName
-	ctxt.xdefine("__image_base__", obj.SDATA, PEBASE)
-
-	ctxt.xdefine("_image_base__", obj.SDATA, PEBASE)
+	if Linkmode == LinkInternal {
+		// some mingw libs depend on this symbol, for example, FindPESectionByName
+		ctxt.xdefine("__image_base__", obj.SDATA, PEBASE)
+		ctxt.xdefine("_image_base__", obj.SDATA, PEBASE)
+	}
 
 	HEADR = PEFILEHEADR
 	if *FlagTextAddr == -1 {