gettime: offset CPU cycle counter by initial value

Should then be safe for the full 2^64 cycles, and we push
the more expensive variable division to later in the run (which
is probably never).

Signed-off-by: Jens Axboe <axboe@fb.com>
gettime: fix compile warning for !ARCH_HAVE_CPU_CLOCK

Signed-off-by: Jens Axboe <axboe@fb.com>