selftests/bpf: introduce bpf_cpu_to_be64 and bpf_be64_to_cpu

test_lwt_seg6local and test_seg6_loop use custom 64-bit endianness
conversion macros. Centralize their definitions in bpf_endian.h in order
to reduce code duplication. This will also be useful when bpf_endian.h
is promoted to an offical libbpf header.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>