um: Use the x86 checksum implementation on 32-bit

When UML is compiled under 32-bit x86, it uses its own copy of
checksum_32.S, which is terribly out-of-date and doesn't support
checksumming unaligned data.

This causes the new "checksum" KUnit test to fail:
./tools/testing/kunit/kunit.py run --kconfig_add CONFIG_64BIT=n --cross_compile i686-linux-gnu- checksum
    KTAP version 1
    # Subtest: checksum
    1..3
    # test_csum_fixed_random_inputs: ASSERTION FAILED at lib/checksum_kunit.c:243
    Expected result == expec, but
        result == 33316 (0x8224)
        expec == 33488 (0x82d0)
    not ok 1 test_csum_fixed_random_inputs
    # test_csum_all_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:267
    Expected result == expec, but
        result == 65280 (0xff00)
        expec == 0 (0x0)
    not ok 2 test_csum_all_carry_inputs
    # test_csum_no_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:306
    Expected result == expec, but
        result == 65531 (0xfffb)
        expec == 0 (0x0)
    not ok 3 test_csum_no_carry_inputs

Sharing the normal implementation in arch/x86/lib both fixes all of
these issues and means any further fixes only need to be done once.

x86_64 already seems to share the same implementation between UML and
"normal" x86.

Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>