soc: nordic: Fix undefined z_arm_platform_init

Fixes #71511
Follow-up to #70977

Update the approach by moving the `PROVIDE` directive to a separate
linker script added using `zephyr_linker_sources()`. This makes the
change more likely to propagate to existing samples which are using
CONFIG_CUSTOM_LINKER_SCRIPT.

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