selftests/bpf: Support cross-endian building

selftests/bpf: Support cross-endian building

Update Makefile build rules to compile BPF programs with target endianness
rather than host byte-order. With recent changes, this allows building the
full selftests/bpf suite hosted on x86_64 and targeting s390x or mips64eb
for example.

Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/880ccc6342cfc4d3c48b44f581e87adfbce2876e.1726475448.git.tony.ambardar@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>