[LLDB] Add tests for PECOFF arm architecture identification

[LLDB] Add tests for PECOFF arm architecture identification

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.)

Differential Revision: https://reviews.llvm.org/D67951

llvm-svn: 372741