bpf: Add kernel test functions for fentry testing

Add few kernel functions with various number of arguments,
their types and sizes for BPF trampoline testing to cover
different calling conventions.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191114185720.1641606-9-ast@kernel.org