ARM: zImage: prevent constant copy+rebuild of lib1funcs.S

The rule to copy this file doesn't have to be forced.  However
lib1funcs.[So] have to be listed amongst the targets.

This prevents zImage from being recreated needlessly.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Dave Martin <dave.martin@linaro.org>
Tested-by: Thomas Abraham <thomas.abraham@linaro.org>