selftest/bpf: Fix profiler test using CO-RE relocation for enums

Instead of hard-coding invalid pids_cgrp_id, use Kconfig to detect the
presence of that enum value and CO-RE to capture its actual value in the
hosts's kernel.

Fixes: 03d4d13f ("selftests/bpf: Add profiler test")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201022202739.3667367-1-andrii@kernel.org