[libc] [llvm] [libc] Disable -march=native in CI to fix sccache poisoning (PR #196560)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

It looks like there's not a good doc page for putting CMake variables. Out of scope for this change, but it would be nice to have, similar to https://llvm.org/docs/CMake.html.

https://github.com/llvm/llvm-project/pull/196560