kbuild: userprogs: avoid duplication of flags inherited from kernel

The duplication makes maintenance harder. Changes need to be done in two
places and the lines will grow overly long.

Use an intermediary variable instead.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20250813-kbuild-userprogs-bits-v1-1-2d9f7f411083@linutronix.de


Signed-off-by: Nathan Chancellor <nathan@kernel.org>