Make more efficient clearing interrupts on all processors at boot time.

Make more efficient clearing interrupts on all processors at boot time.

Without this modification, because of the volatile qualifier, the
compiler does not produce four `` = 0 '' assignments, but code
equivalent to:

	*(volatile uint32_t *)INT_ST_MASK3 = 0;
	*(volatile uint32_t *)INT_ST_MASK2 =
	    *(volatile uint32_t *)INT_ST_MASK3;
	*(volatile uint32_t *)INT_ST_MASK1 =
	    *(volatile uint32_t *)INT_ST_MASK2;
	*(volatile uint32_t *)INT_ST_MASK0 =
	    *(volatile uint32_t *)INT_ST_MASK1;

Anders Gavare reported to Miod Vallat, and he gave me a diff.