vtimer: enable vtimer_gettimeofday as fallback

This is a quick workaround for modules, like *libcoap* requiring the (deprecated) `_gettimeofday()` system call. By declaring `vtimer_gettimeofday()` as a "weak" alias for the _gettimeofday syscall, periph RTC implementations may implement it in a more energy efficient manner. I'm not gonna include any peripheral implementation in this PR, because the whole timer system may change soon anyway.