[RISCV] Implement the TargetLowering::getRegisterByName hook

Summary: The hook should work for any RISC-V register. Non-allocatable registers
do not need to be reserved, for the remaining the hook will only succeed
if you pass clang the -ffixed-xX flag. This builds upon D67185, which
currently only allows reserving GPRs.

Reviewers: asb, lenary

Reviewed By: lenary

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69130