optimized delayNanoseconds

shifting both sides by 16 bit gives pretty good accuracy with fast code:
        movw    r3, #9155
        ldr     r2, .L2
        mul     r0, r3, r0
        lsrs    r0, r0, #1
        umull   r3, r0, r2, r0
        lsrs    r0, r0, #8
.L2:
        .word   144122641

with 984 MHz it allows nsec to be max ~143000  max which is plenty

teensy4/core_pins.h

 static inline void delayNanoseconds(uint32_t nsec)
 {
 	uint32_t begin = ARM_DWT_CYCCNT;
-	uint32_t cycles =  ((uint64_t)F_CPU_ACTUAL * nsec) / 1000000000UL;
+	uint32_t cycles =   ((F_CPU_ACTUAL>>16) * nsec) / (1000000000UL>>16);
 	while (ARM_DWT_CYCCNT - begin < cycles) ; // wait
 }