[linker] In ResolveFromXmlStep, bail if no <linker> element is found.

[linker] In ResolveFromXmlStep, bail if no <linker> element is found.

[merged from monodroid by rodo]

commit 2edecdc39ca55c65de4f57557e5e31b5d432eaa4
Author: Alex Rønne Petersen <alexrp@xamarin.com>
Date:   Mon Jul 1 15:35:59 2013 +0200

    The BlacklistStep pass will feed the ResolveFromXmlStep pass XML files
    that may not necessarily *be* linker descriptor files. This can happen
    if an embedded file happens to be named "Foo.Bar.xml" and an assembly
    called Foo.Bar is referenced in a project. The chances of this happening
    are probably very low, but better safe than sorry.