lib: overflow: Do not define 64-bit tests on 32-bit

lib: overflow: Do not define 64-bit tests on 32-bit

The 64-bit overflow tests will trigger 64-bit division on 32-bit hosts,
which is not currently used anywhere in the kernel, and tickles bugs
in at least Clang 13 and earlier:
https://github.com/ClangBuiltLinux/linux/issues/1636

In reality, there shouldn't be a reason to not build the 64-bit test
cases on 32-bit systems, so these #ifdefs can be removed once the minimum
Clang version reaches 13.

In the meantime, silence W=1 warnings given by the current code:

../lib/overflow_kunit.c:191:19: warning: 's64_tests' defined but not used [-Wunused-const-variable=]
  191 | DEFINE_TEST_ARRAY(s64) = {
      |                   ^~~
../lib/overflow_kunit.c:24:11: note: in definition of macro 'DEFINE_TEST_ARRAY'
   24 |         } t ## _tests[]
      |           ^
../lib/overflow_kunit.c:94:19: warning: 'u64_tests' defined but not used [-Wunused-const-variable=]
   94 | DEFINE_TEST_ARRAY(u64) = {
      |                   ^~~
../lib/overflow_kunit.c:24:11: note: in definition of macro 'DEFINE_TEST_ARRAY'
   24 |         } t ## _tests[]
      |           ^

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202205110324.7GrtxG8u-lkp@intel.com
Fixes: 455a35a6cdb6 ("lib: add runtime test of check_*_overflow functions")
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Vitor Massaru Iha <vitor@massaru.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Tested-by: Daniel Latypov <dlatypov@google.com>
Link: https://lore.kernel.org/lkml/CAGS_qxokQAjQRip2vPi80toW7hmBnXf=KMTNT51B1wuDqSZuVQ@mail.gmail.com
Signed-off-by: Kees Cook <keescook@chromium.org>