sys/util: Stricter bytecpy impl
sys/util: Stricter bytecpy impl

The docstring for this function states that it is guaranteed to perform
a copy byte by byte, but this is not true in general without a
`volatile` storage type on the casted pointer.

Signed-off-by: Chris McDonald <4ca9f429749ea29f0277a76979656ec13beb5513@chromium.org>

finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr