libbpf: fix up global symbol counting logic

Add the same negative ABS filter that we use in VERSIONED_SYM_COUNT to
filter out ABS symbols like LIBBPF_0.8.0.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220518185915.3529475-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>