Add a test case for the change from SVN r372657, and for the preexisting ARM identification.
Add a missing ArchDefinitionEntry for PECOFF/arm64, and tweak the ArmNt case to set the architecture to armv7 (ArmNt never ran on anything lower than that). (This avoids a case where ArchSpec::MergeFrom would override the arch from arm to armv7 and ArchSpec::CoreUpdated would reset the OS to unknown at the same time.)