hwpmc support for Ivy Bridge Xeon

I've tried to update hwpmc by adding support for xeon class of Ivy
bridge processors.

Thanks Jim for pointing me to the correct document. (325462-045US Jan 2013)

I do not have a reference machine to test with. Any help in that
regard would be appreciated.

Here are the diffs against head (245927):
http://www.strugglingcoder.info/patches/hwpmc_ibx.txt

Thanks,
Hiren