[llvm] [RISCV] Add initial assembler/MC layer support for big-endian (PR #146534)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

I'm happy with this as is.

I agree for the need for some kind of "experimental" warning, but I think that's better to come from clang, probably in `RISCVTargetInfo::handleTargetFeatures` - where we also have things like the `+experimental` feature so it can be disabled if need be.

I think this is safe to land as there have been no changes to clang, clang assumes little-endian for RISC-V, and most users will be using clang rather than e.g. `llvm-mc`.

https://github.com/llvm/llvm-project/pull/146534