x86: convert rdtscll() to use __native_read_tsc

Impact: micro-optimization

Is there any reason why x86 rdtscll have to use the out of line
function instead of inline __native_read_tsc()?  native_read_tsc and
__native_read_tsc is essentially the same functions.

Patch to let x86 rdtscll() to use the inline version of read_tsc.

Signed-off-by: Ken Chen <kenchen@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>