[clang] [libc] [llvm] Reapply "[AMDGPU] Use COV6 by default (#118515)" (PR #130963)

[Shilei Tian via llvm-commits]

shiltian wrote:

> The OpenMP runtime doesn't know how to handle `generic` ISAs right?

I think people are working on it?

https://github.com/llvm/llvm-project/pull/130963