soc: nordic: Provide z_arm_platform_init at link time

Nordic SoCs use this hook to execute `SystemInit` as early as possible
after ARM core reset. Previously, `z_arm_platform_init` was defined as
assembly code, which would simply jump to the other function.

However, this extra code can be avoided by using `SystemInit` directly
in place of the `z_arm_platform_init` symbol (whenever it's undefined).
This is now done with a linker script containing a `PROVIDE` directive.

This saves 4 bytes of ROM (0-16 depending on alignment) and also makes
it possible to override `z_arm_platform_init` out of tree, if needed.

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