[RISCV] MC layer support for the instructions added in the privileged spec

Adds support for the instructions added in the RISC-V privileged ISA
(https://content.riscv.org/wp-content/uploads/2017/05/riscv-privileged-v1.10.pdf):
uret, sret, mret, wfi, and sfence.vma.

Note from the committer: I made very minor formatting changes prior to commit, 
which didn't seem worth creating another review round-trip for.

Differential Revision: https://reviews.llvm.org/D40383

Patch by David Craven.

llvm-svn: 320484