[clang] [llvm] [BPF] introduce __attribute__((bpf_fastcall)) (PR #105417)

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eddyz87 wrote:

> The last commit looks fine to me, I didn't look at the rest in detail assuming it was already reviewed.
> 
> One thing I'm curious about is why this is implemented as a function attribute rather than as a calling convention.

Hi @nikic ,

Same question was raised on a previous iteration, please refer to these two comments: [1](https://github.com/llvm/llvm-project/pull/101228#issuecomment-2263848414), [2](https://github.com/llvm/llvm-project/pull/101228#issuecomment-2269797853).

https://github.com/llvm/llvm-project/pull/105417