Add target a target hook to get the register number used by the compact unwind

Add target a target hook to get the register number used by the compact unwind
encoding for the registers it knows about. Return -1 if it can't handle that
register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134202 91177308-0d34-0410-b5e6-96231b3b80d8