ARM: correct handling of features in arch_extension

ARM: correct handling of features in arch_extension

The subtarget information is the ultimate source of truth for the feature set
that is enabled at this point.  We would previously not propagate the feature
information to the subtarget.  While this worked for the most part (features
would be enabled/disabled as requested), if another operation that changed the
feature bits was encountered (such as a mode switch via a .arm or .thumb
directive), we would end up resetting the behaviour of the architectural
extensions.

Handling this properly requires a slightly more complicated handling.  We need
to check if the feature is now being toggled.  If so, only then do we toggle the
features.  In return, we no longer have to calculate the feature bits ourselves.

The test changes are mostly to the diagnosis, which is now more uniform (a nice
side effect!).  Add an additional test to ensure that we handle this case
properly.

Thanks to Nico Weber for alerting me to this issue!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214057 91177308-0d34-0410-b5e6-96231b3b80d8