kunit/usercopy: Disable u64 test on 32-bit SPARC

kunit/usercopy: Disable u64 test on 32-bit SPARC

usercopy of 64 bit values does not work on 32-bit SPARC:

    # usercopy_test_valid: EXPECTATION FAILED at lib/tests/usercopy_kunit.c:209
    Expected val_u64 == 0x5a5b5c5d6a6b6c6d, but
        val_u64 == 1515936861 (0x5a5b5c5d)
        0x5a5b5c5d6a6b6c6d == 6510899242581322861 (0x5a5b5c5d6a6b6c6d)

Disable the test.

Fixes: 4c5d7bc63775 ("usercopy: Add tests for all get_user() sizes")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://lore.kernel.org/r/20250416-kunit-sparc-usercopy-v1-1-a772054db3af@linutronix.de
Signed-off-by: Kees Cook <kees@kernel.org>