[RISCV] Initial support .insn directive for the assembler.

This allows for a custom encoding to be emitted. It can also be
used with inline assembly to allow the custom instruction to be
register allocated like other instructions.

I initially started from SystemZ's implementation, but some of
the formats allow operands to be specified in multiple ways so I
had to add support for matching different operand class lists for
the same format. That implementation is a simplified version of
what is emitted by tablegen for regular instructions.

I've left out the compressed formats. And I haven't supported the
named opcodes like LUI or OP_IMM_32. Those can be added in future
patches.

Documentation can be found here https://sourceware.org/binutils/docs-2.37/as/RISC_002dV_002dFormats.html

Reviewed By: jrtc27, MaskRay

Differential Revision: https://reviews.llvm.org/D108602