[linker] Bump linker submodule to fix #58063 (#2529)

Pick up:

commit ebbd5b492321d092feae425e8f7aefc3c273530e (HEAD -> d15-4, origin/d15-4)
Author: Marek Safar <marek.safar@gmail.com>
Date:   Wed Aug 23 00:44:47 2017 +0200

    Don't crash on any wrong type forwader reference

reference:
https://bugzilla.xamarin.com/show_bug.cgi?id=58063

external/linker

@@ -1 +1 @@
-Subproject commit 03e35128150715b016f0bbc2d1e3502d91c228f5
+Subproject commit ebbd5b492321d092feae425e8f7aefc3c273530e