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

[Matt Arsenault via llvm-commits]

arsenm wrote:

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

I don't understand why that is tied to the version 

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