gettime: print 64-bit variable with ULL

Signed-off-by: Jens Axboe <axboe@kernel.dk>
powerpc: use ATB clock, if it's available

The ATB is more precise, but we don't know if it's available.
Check by forking off a process that attempts to use it, if
we die with SIGILL, then we know it's not there.

Based on code from Steven Noonan.

Signed-off-by: Jens Axboe <axboe@kernel.dk>