[libc] [llvm] [libc] Repurpose `LIBC_GPU_BUILD` option to enable the new format (PR #82848)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

@petrhosek Any objections to this? I understand you're working to get rid of `default` so I'd assume this would conflict.

https://github.com/llvm/llvm-project/pull/82848