fixed core vlpr issue w/ going into sleep modes @ 2 MHz

While writing my new low power library I discovered an issue with the current way of setting up the clocks for 2 MHz and entering VLPR later on in initialization and going into sleep modes. None of the sleep modes will exit with the current setup. If you move the 2 MHz and VLPR setup to after ``` __libc_init_array(); ``` makes it possible to use the sleep modes normally. Also with the early initialization you can't exit VLPR mode to RUN mode either.  

This will be beneficial for the AVR LowPower port and my library so I hope you consider it.