bpf, tests: Add word-order tests for load/store of double words

A double word (64-bit) load/store may be implemented as two successive
32-bit operations, one for each word. Check that the order of those
operations is consistent with the machine endianness.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210809091829.810076-10-johan.almbladh@anyfinetworks.com