Add ARM ERET and HVC virtualisation extension instructions.

Patch by Matthew Wahab.

Change-Id: Iad75f078fbaa4ecc7d7a4820ad9b3930679cbbbb

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222989 91177308-0d34-0410-b5e6-96231b3b80d8