hal_isr: provide facility to turn off GIE when exiting LPM
hal_isr: provide facility to turn off GIE when exiting LPM

The existing BSP430_HAL_ISR_CALLBACK_DISABLE_INTERRUPT does not do this, but
it's required when managing interrupts to ensure events are not lost.  See
also issue #28.

pabigot/bsp430,pabigot/bsp430,pabigot/bsp430,pabigot/bsp430