Add /dev/rtc backend to SystemClock

On devices without /dev/alarm, use the settimeofday() syscall and the
standard RTC_SET_TIME ioctl (which are collectively equivalent to the
ANDROID_ALARM_SET_RTC ioctl).

Change-Id: I3c1d741099e253186e43c9369b62603b214b9c9a
Signed-off-by: Greg Hackmann <ghackmann@google.com>