selftests/bpf: Fix bind program for big endian systems

Without this fix, the bind4 and bind6 programs will reject bind attempts
on big endian systems. This patch ensures that CI tests pass for the
s390x architecture.

Signed-off-by: Jordan Rife <jrife@google.com>
Link: https://lore.kernel.org/r/20240429214529.2644801-2-jrife@google.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>