soc: nordic: Resolve z_arm_platform_init in the linker generator

Fixes #72673
Follow-up to #70977 and #71590

The CMake linker generator doesn't have an API equivalent to `PROVIDE`,
but the existing `zephyr_linker_symbol()` function should do just fine.
It still lets us set `z_arm_platform_init = SystemInit` and thus keep
the reduced ROM space.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>