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

[via cfe-commits]

eddyz87 wrote:

@AaronBallman , @efriedma-quic , could you please check my last comment [here](https://github.com/llvm/llvm-project/pull/101228#issuecomment-2263848414)?

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