[PowerPC] Add DWARF numbers for CA (XER), etc.

For registers that have DWARF numbers (like CA, which is really part of XER),
add them. Also, RM is not an SPR, and the declaration hack (where it is
declared as an SPR with an arbitrary number) is not needed, so just declare it
as a register.

NFC; although CA's register number will be needed when stackmap/patchpoint
support is added.

llvm-svn: 225800