selftests/bpf: fix "alu with different scalars 1" on s390

BPF_LDX_MEM is used to load the least significant byte of the retrieved
test_val.index, however, on big-endian machines it ends up retrieving
the most significant byte.

Change the test to load the whole int in order to make it
endianness-independent.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>