selftests/bpf: add CO-RE relocs ints tests

Add various tests validating handling compatible/incompatible integer
types.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>