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 <thomas.weissschuh@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250505-nolibc-kselftest-harness-v4-6-ee4dd5257135@linutronix.de


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>