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

[Vladimir Vereschaka via cfe-commits]

vvereschaka wrote:

@eddyz87 ,

just create a new PR with the changes and make the cross references between this PR and the new one to let people easy find the original PR when necessary.

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