cmd/ld: fix "_image_base__ not defined" problem for cmd/8l.

Fixes #6431.

Change suggested by kin.wilson.za.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13314048

src/cmd/ld/pe.c

@@ -151,6 +151,7 @@ peinit(void)
 
 	// some mingw libs depend on this symbol, for example, FindPESectionByName
 	xdefine("__image_base__", SDATA, PEBASE);
+	xdefine("_image_base__", SDATA, PEBASE);
 }
 
 static void