kernel/timeout: Add absolute timeout APIs

Add support for "absolute" timeouts, which are expressed relative to
system uptime instead of deltas from current time.  These allow for
more race-resistant code to be written by allowing application code to
do a single timeout computation, once, and then reuse the timeout
value even if the thread wakes up and needs to suspend again later.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>