Disable all maskable interrupts on core 1 during API calls.  This brings
two main advantages:

1. It means API calls are now always ISR-safe and can be used everywhere
   in core 1 code.  This is mostly interesting to l0dables as Pycardium
   should not need to do this.

2. It allows Epicardium to halt the clock for core 1 without fear as we
   have observed problems with doing this when core 1 is currently
   executing instructions that touch memory.  Now a synchronous call
   from core 1 will guarantee that it is currently waiting in a WFE and
   no other ISRs could be potentially running.