linker: Move sgstubs section to the very end and fix for nrf91

Add ifdefs to handle the nrf91 case. This change will dynamically
place and size the NSC region according to nrf91 HW limitations.

Add Cmake check of NSC offset if manually set.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>