[COREV] Add Hardware Loop Extension

Created by: flip1995

I have some open questions (see comments below), but this PR should be good for an initial review.

r? @simonpcook

(cc @jeremybennett, @MaryBennett, @jessicamills, @pietraferreira)

This PR is the llvm equivalent to https://github.com/openhwgroup/corev-binutils-gdb/pull/2