selftests/bpf: get rid of -D__x86_64__

-D__x86_64__ workaround was used to make /usr/include/features.h
to follow expected path through the system include headers.
This is not portable.
Instead define dummy stubs.h which is used by 'clang -target bpf'

Fixes: 6882804c

 ("selftests/bpf: add a test for overlapping packet range checks")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>