[llvm] [LLVM][MC][DecoderEmitter] Add support to specialize decoder per bitwidth (PR #154865)

[Rahul Joshi via llvm-commits]

jurahul wrote:

@s-barannikov this is ready for another round of review. The delta is that there are no longer some default implementations of `InsnBitWidth` in MCDecoder.h, so individual targets need to provide them according to their usage. This enables, for example, for RISCV to use uint64_t for 48 bit instructions. 

https://github.com/llvm/llvm-project/pull/154865