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

[via cfe-commits]

eddyz87 wrote:

@AaronBallman, @efriedma-quic, could you please comment?
We are eager to use this feature on the BPF side (e.g. [here](https://lore.kernel.org/bpf/CAADnVQJDki9GCxDAaGJWb+HrKT2EnzYXM8K3238XxPtHkhU0Ag@mail.gmail.com/) and landing it to the `main` would simplify the cooperation.

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