[llvm-mc] Add -M to replace -riscv-no-aliases and -riscv-arch-reg-names

In objdump, many targets support `-M no-aliases`.  Instead of having a
`-*-no-aliases` for each target when LLVM adds the support, it makes more sense
to introduce objdump style `-M`.

-riscv-arch-reg-names is removed. -riscv-no-aliases has too many uses and thus is retained for now.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D103004