arm: cortex-m: add an option to trap unaligned access

Cortex-M mainline cores have an option to generate a fault on word and
halfword unaligned access [1], this patch adds a Kconfig option for
enabling the feature.

[1] https://developer.arm.com/documentation/dui0552/a/cortex-m3-peripherals/system-control-block/configuration-and-control-register



Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>