selftests/bpf: Fix test_core_reloc_mods on big-endian machines

This is the same as commit d164dd9a ("selftests/bpf: Fix
test_core_autosize on big-endian machines"), but for
test_core_reloc_mods.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211026010831.748682-7-iii@linux.ibm.com