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>

tools/testing/selftests/bpf/verifier/value_ptr_arith.c

@@ -183,7 +183,7 @@
 	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
 	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
 	BPF_EXIT_INSN(),
-	BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_0, 0),
+	BPF_LDX_MEM(BPF_W, BPF_REG_1, BPF_REG_0, 0),
 	BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0, 3),
 	BPF_MOV64_IMM(BPF_REG_2, 0),
 	BPF_MOV64_IMM(BPF_REG_3, 0x100000),