Add check for invariant TSC on x86 and use TSC is default clock if reliable

TSC is by far the fastest clock we can use. Check the CPUID bits for
whether it is both constant rate AND synced across cores. If it is,
we can use it as our default clock source.

Fio will default to this clock source on x86 if no other clock source
is specifically given with clocksource= in the job file.

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