Ensure system PLLs on Juno are locked within MCC timeout

The current approach to resetting the PLLs, which is to use a pre-main constructor function, is not infallible - if we statically allocate enough data, eventually the time it takes for the C runtime to initialize exceeds the amount of time that the motherboard microcontroller gives us to ensure the system PLLs are locked.

The workaround for this is to instead wrap the core's reset handler. This way we ensure that the PLLs have been reset before we do any time- consuming work.