[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Petr Penzin via llvm-commits]

ppenzin wrote:

As previously requested, I've taken the contents of @mgudim's branch and made a PR stack:

- PR 1/5: https://github.com/llvm/llvm-project/pull/170606
- PR 2/5: https://github.com/llvm/llvm-project/pull/170607
- PR 3/5: https://github.com/llvm/llvm-project/pull/170609
- PR 4/5: https://github.com/llvm/llvm-project/pull/170610
- PR 5/5: https://github.com/llvm/llvm-project/pull/170611

I've omitted one "fix a test' commit and a couple of commits are swapped by accident. It is also rebased on a more recent `main`. PR 2 has clean CI and can be probably extracted out, though might require additional DWARF verification.

https://github.com/llvm/llvm-project/pull/90819