mini-os: x86: ensure time is correct once init_time returns

Before, time values weren't correct until the first timer interrupt,
leading to output such as this from test.c:

Periodic thread started.
T(s=8734 us=790808)
T(s=1413543796 us=487564)
T(s=1413543797 us=487867)

Signed-off-by: Thomas Leonard <talex5@gmail.com>