Refactor ARM subarchitecture parsing

Re-commit of a patch to rework the triple parsing on ARM to a more sane
model.

Patch by Gabor Ballabas.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213367 91177308-0d34-0410-b5e6-96231b3b80d8