timerfd: add alarm timers

Add support for clocks CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM.

Change-Id: Iafc8445d3d7ffb35110c860f1607bf03f1edb895
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Dan Trifan <jimsth@gmail.com>