[RISCV] Add Option for Printing Architectural Register Names

Summary:
This is an option primarily to use during testing. Instead of always
printing registers using their ABI names, this allows a user to request they
are printed with their architectural name.

This is then used in the register constraint tests to ensure the mapping
between architectural and abi names is correct.

Reviewers: asb, luismarques

Reviewed By: asb

Subscribers: pzheng, hiraditya, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, s.egerton, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65950

llvm-svn: 371531