bpf: support for kernel_helper attributes

[jameshilliard]

GCC currently uses kernel_helper attributes to implement bpf helper functions.

These have the advantage of working at any optimization level as well unlike llvm style helpers.

It would be helpful to have support for these kernel_helper attributes in llvm as they should be more reliable and allow for unified llvm/GCC bpf helper definitions.

See details:
https://lore.kernel.org/bpf/87v8s260j1.fsf@oracle.com/
https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/config/bpf/bpf.cc#L838-L846

Cc: @jemarch