[clang] [DRAFT][RISCV] Emit arch string macro to facilitate ASM programming (PR #85063)

[Nemanja Ivanovic via cfe-commits]

nemanjai wrote:

This is a suggestion born out of a request from a user to have a way to emit the ISA attributes into objects produced from pre-processed asm files. Perhaps there is already a method to do this that I'm not aware of, but if not, I think this is a convenient and lightweight way to do so.

https://github.com/llvm/llvm-project/pull/85063