hwpmc support for Ivy Bridge Xeon

[hiren panchasara]

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