libc: Move alarm implementation to itimer class, Add setitimer/getitimer

Move static variables for alarm and related function to itimer class.
itimer class offers interfaces to implement setitimer/getitimer, so added setitimer/getitimer.
Now alarm rewrite as a just a wrapper function to call setitimer.

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>