Add Thumb HVC and ERET virtualisation extension instructions.

Patch by Matthew Wahab.

Change-Id: I131f71c1150d5fa797066a18e09d526c19bf9016

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222990 91177308-0d34-0410-b5e6-96231b3b80d8