[RISCV] Add assembly mnemonic spell checking

Summary:
This allows the assembler to suggest alternative assembly mnemonics when
an invalid one has been provided.

Reviewers: asb, lenary, lewis-revill

Reviewed By: asb

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69894