scripts: Add align for empty _app_smem region

When _app_smem region is empty, alignment is also needed. If there
is no alignment, the _app_smem_start used by arm mpu can be lower
than __rodata_region_end, and this two regions can overlap.

The Armv8-R aarch64 MPU does not allow overlapped regions.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>