selftests: harness: Remove dependency on libatomic

__sync_bool_compare_and_swap() is deprecated and requires libatomic on
GCC. Compiler toolchains don't necessarily have libatomic available, so
avoid this requirement by using atomics that don't need libatomic.

Signed-off-by: Thomas Weißschuh <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Acked-by: Shuah Khan <[email protected]>
Link: https://lore.kernel.org/r/20250505-nolibc-kselftest-harness-v4-6-ee4dd5257135@linutronix.de
Signed-off-by: Thomas Weißschuh <[email protected]>

Author: t-8ch

tools/testing/selftests/kselftest_harness.h

@@ -439,12 +439,12 @@
 		} \
 		if (child == 0) { \
 			if (_metadata->setup_completed && !fixture_name##_teardown_parent && \
-					__sync_bool_compare_and_swap(teardown, false, true)) \
+					!__atomic_test_and_set(teardown, __ATOMIC_RELAXED)) \
 				fixture_name##_teardown(_metadata, self, variant->data); \
 			_exit(0); \
 		} \
 		if (_metadata->setup_completed && fixture_name##_teardown_parent && \
-				__sync_bool_compare_and_swap(teardown, false, true)) \
+				!__atomic_test_and_set(teardown, __ATOMIC_RELAXED)) \
 			fixture_name##_teardown(_metadata, self, variant->data); \
 		munmap(teardown, sizeof(*teardown)); \
 		if (self && fixture_name##_teardown_parent) \