selftests/bpf: Add big-endian support to the ldsx test

Prepare the ldsx test to run on big-endian systems by adding the
necessary endianness checks around narrow memory accesses.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230919101336.2223655-4-iii@linux.ibm.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>