Remove use of multiple reg index enums by RegisterContextPOSIX_arm64

Summary:
This patch removes dependence of RegisterContextPOSIX_arm64 on register number enums defined in lldb-arm64-register-enums.h.
RegisterContextPOSIX_arm64 makes use of helper functions to access register numbers defined in RegisterInfos_arm64.h via RegisterInfosPOSIX_arm64.

Reviewers: labath

Reviewed By: labath

Subscribers: emaste, kristof.beyls, arphaman, danielkiss, lldb-commits

Differential Revision: https://reviews.llvm.org/D83753