patch: enable MEM_LOAD_UOPS_LLC_HIT_RETIRED on sandy bridge xeon

[Adrian Chadd]

Attempt #2:

http://people.freebsd.org/~adrian/netflix/20130816-hwpmc-sbx-1.diff

I've verified that I get counter events from this! But I need to load up a
system more to trigger contention to validate that they're doing the right
thing.

Unfortunately there's an erratum on this which we need to add in the hack
for:

http://software.intel.com/en-us/articles/performance-monitoring-on-intel-xeon-processor-e5-family

.. sigh.



-adrian