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

[Vladimir Vereschaka via cfe-commits]

vvereschaka wrote:

Hi @eddyz87 ,

there are few failed `fastcall` tests on `[llvm-clang-x86_64-expensive-checks-ubuntu](https://lab.llvm.org/buildbot/#/builders/187) also

https://lab.llvm.org/buildbot/#/builders/187/builds/509

* LLVM::bpf-fastcall-regmask-1.ll
* LLVM::bpf-fastcall-1.ll
* LLVM::bpf-fastcall-3.ll
* LLVM::bpf-fastcall-2.ll

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