Update PlatformDarwin to understand how armv7m / armv7em relate
to the other armv7 types as far as compatibility/interchangability
goes.
<rdar://problem/13361372> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@176684 91177308-0d34-0410-b5e6-96231b3b80d8