cmake: linker: evaluate CONFIG_LINKER_ITERABLE_SUBALIGN in CMake

Evaluate CONFIG_LINKER_ITERABLE_SUBALIGN in CMake instead of in the
linker script.

It cannot be assumed that a linker generator implementation is
pre-processed and therefore Kconfig settings cannot be used in those
but must be evaluated in CMake.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>