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: 455a35a6 ("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.comSigned-off-by: Kees Cook <keescook@chromium.org>

lib/overflow_kunit.c

@@ -91,6 +91,7 @@ DEFINE_TEST_ARRAY(u32) = {
 	{-4U, 5U, 1U, -9U, -20U, true, false, true},
 };
 
+#if BITS_PER_LONG == 64
 DEFINE_TEST_ARRAY(u64) = {
 	{0, 0, 0, 0, 0, false, false, false},
 	{1, 1, 2, 0, 1, false, false, false},
@@ -114,6 +115,7 @@ DEFINE_TEST_ARRAY(u64) = {
 	 false, true, false},
 	{-15ULL, 10ULL, -5ULL, -25ULL, -150ULL, false, false, true},
 };
+#endif
 
 DEFINE_TEST_ARRAY(s8) = {
 	{0, 0, 0, 0, 0, false, false, false},
@@ -188,6 +190,8 @@ DEFINE_TEST_ARRAY(s32) = {
 	{S32_MIN, S32_MIN, 0, 0, 0, true, false, true},
 	{S32_MAX, S32_MAX, -2, 0, 1, true, false, true},
 };
+
+#if BITS_PER_LONG == 64
 DEFINE_TEST_ARRAY(s64) = {
 	{0, 0, 0, 0, 0, false, false, false},
 
@@ -216,6 +220,7 @@ DEFINE_TEST_ARRAY(s64) = {
 	{-128, -1, -129, -127, 128, false, false, false},
 	{0, -S64_MAX, -S64_MAX, S64_MAX, 0, false, false, false},
 };
+#endif
 
 #define check_one_op(t, fmt, op, sym, a, b, r, of) do {		\
 	t _r;							\
@@ -650,6 +655,7 @@ static struct kunit_case overflow_test_cases[] = {
 	KUNIT_CASE(s16_overflow_test),
 	KUNIT_CASE(u32_overflow_test),
 	KUNIT_CASE(s32_overflow_test),
+/* Clang 13 and earlier generate unwanted libcalls on 32-bit. */
 #if BITS_PER_LONG == 64
 	KUNIT_CASE(u64_overflow_test),
 	KUNIT_CASE(s64_overflow_test),