[clang] [libc][nfc] Use common implementation of read_first_lane_u64 (PR #131027)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 approved this pull request.

This means that `nvptxintrin.h` and `amdgpuintrin.h` can't be included standalone, but I'm not sure it's a big deal since we already define a lot of the common functionality in the `gpurintrin.h` header.

https://github.com/llvm/llvm-project/pull/131027