tools/nolibc: Replace ifdef with if defined() in sys.h

Thomas has requested that if defined() be used in place of ifdef but
currently ifdef is used consistently in sys.h. Update all the instances of
ifdef to if defined().

Suggested-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250703-arm64-gcs-vfork-exit-v3-1-1e9a9d2ddbbe@kernel.org


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