libbpf: Introduce errstr() for stringifying errno

Add function errstr(int err) that allows converting numeric error codes
into string representations.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241111212919.368971-2-mykyta.yatsenko5@gmail.com