libbpf: don't define CC and AR

We are already including tools/scripts/Makefile.include which correctly
handles CROSS_COMPILE, no need to define our own vars.

See related commit 7ed1c190

 ("tools: fix cross-compile var clobbering")
for more details.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>