[clang] [llvm] [RISC-V] Add CSR read/write builtins (PR #85091)

[Wang Pengcheng via cfe-commits]

wangpc-pp wrote:

I support adding these builtins personally, but I think we need more discussions on the design.
We can achieve the same thing via inline assemblies, that's true. But, from the compiler side, inline assemblies are kind of barriers, we can't do a lot of optimizations/reorderings if inline assemblies exist. If we make it a builtin, these limitations can be loosed I think.

https://github.com/llvm/llvm-project/pull/85091