[clang] [llvm] [BPF] introduce `__attribute__((bpf_fastcall))` (PR #101228)

[via llvm-commits]

https://github.com/yonghong-song approved this pull request.

I also tried a few examples and it looks good to me. Only a few minor comments.

https://github.com/llvm/llvm-project/pull/101228