Fix ARM triple parsing

The triple parser should only accept existing architecture names
when the triple starts with armv, armebv, thumbv or thumbebv.

Patch by Gabor Ballabas.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222129 91177308-0d34-0410-b5e6-96231b3b80d8