move getRegisterNumbering() to out of ARMBaseRegisterInfo into the helper

move getRegisterNumbering() to out of ARMBaseRegisterInfo into the helper
functions in ARMBaseInfo.h so it can be used in the MC library as well.
For anything bigger than this, we may want a means to have a small support
library for shared helper functions like this. Cross that bridge when we
come to it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114016 91177308-0d34-0410-b5e6-96231b3b80d8