[RISCV] Support llvm-objdump -M no-aliases and -M numeric

Summary:
Now that llvm-objdump allows target-specific options, we match the
`no-aliases` and `numeric` options for RISC-V, as supported by GNU objdump.

This is done by overriding the variables used for the command-line options, so
that the command-line options are still supported.

This patch updates all tests using `llvm-objdump -riscv-no-aliases` to use
`llvm-objdump -M no-aliases`.

Reviewers: luismarques, asb

Reviewed By: luismarques, 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/D66139

llvm-svn: 371534