selftests/bpf: add __uint and __type macro for BTF-defined maps

Add simple __uint and __type macro that hide details of how type and
integer values are captured in BTF-defined maps.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>