[RISCV] Move FRM parsing in the assembler to a custom operand parser.

[RISCV] Move FRM parsing in the assembler to a custom operand parser.

Previously we parsed it as a symbol, and then accessed the name.
This works, but seems like an unusual way to do it.

This adds a custom parser that looks for an Identifier token containing
one of the valid FRM strings.

Differential Revision: https://reviews.llvm.org/D142833