overflow: disable failing tests for older clang versions

Building the overflow kunit tests with clang-11 fails with:

$ ./tools/testing/kunit/kunit.py run --arch=arm --make_options LLVM=1 \
overflow
...
ld.lld: error: undefined symbol: __mulodi4
...

Clang 11 and earlier generate unwanted libcalls for signed output,
unsigned input.

Disable these tests for now, but should these become used in the kernel
we might consider that as justification for dropping clang-11 support.
Keep the clang-11 build alive a little bit longer.

Avoid -Wunused-function warnings via __maybe_unused.

Link: ClangBuiltLinux#1711
Link: llvm/llvm-project@3203143
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

lib/overflow_kunit.c

@@ -254,6 +254,7 @@ static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
 	check_one_op(t, fmt, mul, "*", p->b, p->a, p->prod, p->p_of);	\
 }									\
 									\
+__maybe_unused								\
 static void n ## _overflow_test(struct kunit *test) {			\
 	unsigned i;							\
 									\
@@ -716,12 +717,23 @@ static struct kunit_case overflow_test_cases[] = {
 	KUNIT_CASE(u32_u32__u32_overflow_test),
 	KUNIT_CASE(s32_s32__s32_overflow_test),
 /* Clang 13 and earlier generate unwanted libcalls on 32-bit. */
-#if BITS_PER_LONG == 64
+#if defined(CONFIG_CC_IS_GCC) || \
+	(defined(CONFIG_CC_IS_CLANG) && \
+		(__clang_major__ > 13 || BITS_PER_LONG == 64))
 	KUNIT_CASE(u64_u64__u64_overflow_test),
 	KUNIT_CASE(s64_s64__s64_overflow_test),
 #endif
-	KUNIT_CASE(u32_u32__u8_overflow_test),
+/*
+ * Clang 11 and earlier generate unwanted libcalls for signed output, unsigned
+ * input.
+ * https://github.com/llvm/llvm-project/commit/3203143f1356a4e4e3ada231156fc6da6e1a9f9d
+ */
+#if defined(CONFIG_CC_IS_GCC) || \
+	(defined(CONFIG_CC_IS_CLANG) && \
+		(__clang_major__ > 11 || BITS_PER_LONG == 64))
 	KUNIT_CASE(u32_u32__int_overflow_test),
+#endif
+	KUNIT_CASE(u32_u32__u8_overflow_test),
 	KUNIT_CASE(u8_u8__int_overflow_test),
 	KUNIT_CASE(int_int__u8_overflow_test),
 	KUNIT_CASE(shift_sane_test),