close #44: work around CPU42
close #44: work around CPU42

This bug was demonstrated in the following code.  mspgcc is not going be
fixed anytime soon, and other toolchains may have the same problem.

BSP430_CORE_ENABLE_INTERRUPT();
while (1) {
  unsigned int t0, t1;
  BSP430_CORE_DISABLE_INTERRUPT();
  t0 = TA0R;
  t1 = TA0R;
  BSP430_CORE_ENABLE_INTERRUPT();
  printf("Time difference %u\n", t1-t0);
}

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