libbpf: attempt to load kernel BTF from sysfs first

Add support for loading kernel BTF from sysfs (/sys/kernel/btf/vmlinux)
as a target BTF. Also extend the list of on disk search paths for
vmlinux ELF image with entries that perf is searching for.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>