nrf: fix off-by-one in modulo of runtime.ticks

This code:
    foo & 0xffffff
Is equivalent to this code:
    foo % 0x1000000
However, to drop the high 8 bits, this calculation was used:
    foo % 0xffffff
This is far more expensive (and incorrect), as it needs an actual modulo
operation which increases code size and probably reduces speed on a
Cortex-M4 and needs library functions for a Cortex-M0 increasing code
size by a much bigger amount.

src/runtime/runtime_nrf.go

@@ -75,7 +75,7 @@ var (
 // handling the overflow event.
 func ticks() timeUnit {
 	rtcCounter := uint32(nrf.RTC0.COUNTER)
-	offset := (rtcCounter - rtcLastCounter) % 0xffffff // change since last measurement
+	offset := (rtcCounter - rtcLastCounter) & 0xffffff // change since last measurement
 	rtcLastCounter = rtcCounter
 	timestamp += timeUnit(offset) // TODO: not precise
 	return timestamp