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

[via llvm-commits]

eddyz87 wrote:

There was a failure for expansive checks:
https://lab.llvm.org/buildbot/#/builders/187/builds/509

I reverted the commit, will investigate.

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