[clang] [clang][RISCV] Support `norelax` attribute for RISCV (PR #115981)

[Sam Elliott via cfe-commits]

lenary wrote:

Two comments:
- I don't think the c-api-doc change is going to be accepted at this rate
- One of the nice things about the monorepo is that you can do a Clang+Backend change like what this requires in a single PR, rather than committing it bit-by-bit. Seeing the entire change in one place makes reviewing the changes a lot easier, even if they are larger.

https://github.com/llvm/llvm-project/pull/115981