Optimize delayMicroseconds() on Teensy-LC

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1

teensy3/pins_teensy.c

 	 //systick_istatus = istatus & SCB_ICSR_PENDSTSET ? 1 : 0;
 	if ((istatus & SCB_ICSR_PENDSTSET) && current > 50) count++;
 	current = ((F_CPU / 1000) - 1) - current;
+#if defined(KINETISL) && F_CPU == 48000000
+	return count * 1000 + ((current * (uint32_t)87381) >> 22);
+#elif defined(KINETISL) && F_CPU == 24000000
+	return count * 1000 + ((current * (uint32_t)174763) >> 23);
+#endif
 	return count * 1000 + current / (F_CPU / 1000000);
 }