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

[Shilei Tian via cfe-commits]

shiltian wrote:

> > AFAIK it has nothing fancy for runtime.
> 
> The `libc` CMake has a variable that sets it to 5, might be worth bumping that up since it's actively tested. It's in the `prepare_libc_gpu_build.cmake`.

Done.

https://github.com/llvm/llvm-project/pull/118515