selftests/bpf: Add read_build_id function

Adding read_build_id function that parses out build id from
specified binary.

It will replace extract_build_id and also be used in following
changes.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230331093157.1749137-3-jolsa@kernel.org


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