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

[Aaron Ballman via cfe-commits]

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

Clang bits LGTM; accepting but please wait to land until someone accepts the LLVM changes as well.

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