[LoongArch] Add basic support to Disassembler

With the addition of disassembler now we can do instructions 'round-trip' test
that assembles `.s` to obj with `llvm-mc` and disassembles it with `llvm-objdump`
to check instruction mnemonics.

Reviewed By: xen0n, MaskRay

Differential Revision: https://reviews.llvm.org/D120477