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

[Aaron Ballman via llvm-commits]

AaronBallman 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.

Yup, this is the typical process (GitHub doesn't let you reopen a merged PR).

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