selftests/bpf: Add fexit tests for BPF trampoline

Add fexit tests for BPF trampoline that checks kernel functions
with up to 6 arguments of different sizes and their return values.

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/20191114185720.1641606-11-ast@kernel.org