Remove code duplication from RegisterContextPOSIX_*

Summary:
This patch aims to remove multiple copies of GetByteOrder() and ConvertRegisterKindToRegisterNumber used in various versions of RegisterContextPOSIX_*.

Both register implementations are move to RegisterContext class which is parent of RegisterContextPOSIX_* classes.

Built and tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabihf targets.

Reviewers: labath

Reviewed By: labath

Subscribers: wuzish, nemanjai, kristof.beyls, kbarton, atanasyan, lldb-commits

Differential Revision: https://reviews.llvm.org/D80104