arch: arm: ensure shared mem and app sram MPU regions don't overlap

This commit contributes a patch to the Arm Cortex-M linker
script, which guarantees that the linker sections for shared
memory and the application memory will have sufficient padding
in between, so that the latter will start from an address that
is 32-byte aligned. This is required for ensuring that the MPU
regions defined using the start and end addresses of the two
sections will not overlap. The patch targets ARMv8-M MPU with
no requirement for power-of-two alignment and size.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>