[clang] [RISCV] Support -march=native (PR #215939)

[Craig Topper via cfe-commits]

topperc wrote:

> Hmmm, I thought we have implemented already. Do we have a sentence in toolchain-convention repo? We may need to write this down.

Doesn't look like we have -mcpu=native or -mtune=native in toolchain conventions either. @kito-cheng does gcc implement any of these today?

https://github.com/llvm/llvm-project/pull/215939