linker: move last section id constant to c-code

Move creation of last section id from ld linker script LONG() usage to
C code with last section attribute.

The use of `LONG()` works correctly with ld but lld emits a warning
because .last_section section is not allocated as there are no matching
input sections and discards the `LONG()` call, meaning the last section
identifier will not be present in the flash.
> ld.lld: warning: ignoring memory region assignment for
>                             non-allocatable section '.last_section'

Placing the last section id in `.last_section` in C code makes lld
allocate the memory for the id and thereby create the output section
with the correct output.

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