timers-urgent-2021-05-16

  - Use the ALARM feature check in the alarmtimer core code insted of
    the old method of checking for the set_alarm() callback. Drivers
    can have that callback set but the feature bit cleared. If such
    a RTC device is selected then alarms wont work.

  - Use a proper define to let the preprocessor check whether Hyper-V VDSO
    clocksource should be active. The code used a constant in an enum with
    #ifdef, which evaluates to always false and disabled the clocksource
    for VDSO.