selftests/bpf: Add a test for enum64 value relocations

Add a test for enum64 value relocations.
The test will be skipped if clang version is 14 or lower
since enum64 is only supported from version 15.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062718.3726307-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>