selftests/bpf: Test BTF's handling of endianness

Add selftests juggling endianness back and forth to validate BTF's handling of
endianness convertions internally.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200929043046.1324350-4-andriin@fb.com