bsps/arm: Fix ARMv7-M CPU counter

Read the current counter value again after we know that we had an
underflow.

Update #3456.

bsps/arm/include/bsp/clock-armv7m.h

@@ -59,6 +59,7 @@ static uint32_t _ARMV7M_Clock_counter(ARMV7M_Timecounter *tc)
   ticks = tc->ticks;
 
   if ((systick->csr & ARMV7M_SYSTICK_CSR_COUNTFLAG) != 0) {
+    counter = systick->cvr;
     ticks += interval;
     tc->ticks = ticks;
   }