multiple fixes for stm32f1xx RTC clock

- compile issues because of missing RTC_MAGIC #defines
- missing functionality based on RTC_MAGIC in RTC based on stm32_rtcounter.c
- IRQ setup from up_rtc_initialize was later reset in up_irqinitialize
- write access to backup registers without enabling access to backup domain
- possible races in set/cancel alarm
tested with STM32F103C8 only
device now wakes up from forced STANDBY mode by alarm