Import atomics code from MCU Driver HAL

This commit copies code from the MCU Driver HAL, that originally comes
from Mbed OS. While doing so, the following modifications have been
made:

* "mbed_..." and "core_util" prefixes has been changed to "iotsdk_..."
* Toolchain/platform specific utility code has been imported into a
separate header file.

Signed-off-by: Krzysztof Stachowiak <krzysztof.stachowiak2@arm.com>