arch: arm: mpu: Fix alignment check for iccarm

The IAR C/C++ compiler can't resolve a cast on a pointer
to a constant expression. This is used in the
_ARCH_MEM_PARTITION_ALIGN_CHECK macro.

This fix checks if an IAR compiler is used and disables the
start-alignment check if it is.

This was already fixed for v7 in rev d34d554d.

Signed-off-by: Lars-Ove Karlsson <lars-ove.karlsson@iar.com>