These instructions aren't universally available, but depend on a specific
extension to the normal ARM architecture (rather than, say, v6/v7/...) so a new
feature is appropriate.
This also enables the feature by default on A-class cores which usually have
these extensions, to avoid breaking existing code and act as a sensible
default.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179171 91177308-0d34-0410-b5e6-96231b3b80d8
