selftests/bpf: fix "valid read map access into a read-only array 1" on s390

This test looks up a 32-bit map element and then loads it using a 64-bit
load. This does not work on s390, which is a big-endian machine.

Since the point of this test doesn't seem to be loading a smaller value
using a larger load, simply use a 32-bit load.

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