[clang] [libc] [llvm] [openmp] [libc] Rework the GPU build to be a regular target (PR #81921)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Tested this one a few machines and it works as expected after some final tweaks.

FYI @jplehr and @Artem-B, this will change the CMake configuration required for building and testing on the build bots. The new expected way to test each one respectively would be the following
```
-DLLVM_LIBC_RUNTIME_TARGETS=amdgcn-amd-amdhsa;nvptx64-nvidia-cuda
ninja check-libc-amdgcn-amd-amdhsa
ninja check-libc-nvptx64-nvidia-cuda
```
I think specifically there are two CMake issues that might make this annoying. I would like to be able to just do `ninja check-libc` and have it check all of them, but for the bots it's likely enough to just check specifically which one we're interested in.

https://github.com/llvm/llvm-project/pull/81921