selftests/bpf: Fix test_core_autosize on big-endian machines

The "probed" part of test_core_autosize copies an integer using
bpf_core_read() into an integer of a potentially different size.
On big-endian machines a destination offset is required for this to
produce a sensible result.

Fixes: 888d83b9 ("selftests/bpf: Validate libbpf's auto-sizing of LD/ST/STX instructions")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210812224814.187460-1-iii@linux.ibm.com