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

[Nemanja Ivanovic via cfe-commits]

nemanjai wrote:

Ah, I missed the fact that there is an option `-riscv-add-build-attributes` that the clang driver passes when invoking `cc1as`.  Perhaps that option should default to `true`? Thanks and sorry for the noise.

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