teensy40: init RTC and use ARM cycle counter for improved SysTick accuracy

src/runtime/runtime_mimxrt1062.go

@@ -108,6 +108,7 @@ func initPeripherals() {
 
 	enableTimerClocks() // activate GPT/PIT clock gates
 	initSysTick()       // enable SysTick
+	initRTC()           // enable real-time clock
 
 	enablePinClocks() // activate IOMUXC(_GPR)/GPIO clock gates
 	initPins()        // configure GPIO