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

[Jan Patrick Lehr via llvm-commits]

https://github.com/jplehr approved this pull request.

I certainly want this option from a usability point of view as the new syntax is quite cumbersome.

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