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

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eddyz87 wrote:

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

@AaronBallman , thank you for the review!
On the LLVM side the changes are only for BPF backend and these were already approved by @yonghong-song, or do you have someone else in mind?

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