Adapt CatenaStm32L0Rtc::SleepForAlarm() for new STANDBY discipline

[terrillmoore]

Although we won't use it, SleepForAlarm(SleepMode::Standby) should enter true STANDBY. We're changing the clients that used the older code.

[terrillmoore]

On reflection, this would be bad. SleepForAlarm(SleepMode::Standby) should always do a STOP, because people will expect it to return. If we need a true hibernate, we'll add StandbyForAlarm(), and mark it as __attribute__(noreturn).