x86-64: Mark rdtsc as sync only for netburst, not for core2

x86-64: Mark rdtsc as sync only for netburst, not for core2

On the Core2 cpus, the rdtsc instruction is not serializing (as defined
in the architecture reference since rdtsc exists) and due to the deep
speculation of these cores, it's possible that you can observe time go
backwards between cores due to this speculation. Since the kernel
already deals with this with the SYNC_RDTSC flag, the solution is
simple, only assume that the instruction is serializing on family 15...

The price one pays for this is a slightly slower gettimeofday (by a
dozen or two cycles), but that increase is quite small to pay for a
really-going-forward tsc counter.

Backport by Chris Wright.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>