kernel - Add workaround for errata #721 on AMD cpus (found by Matt Dillon)

kernel - Add workaround for errata #721 on AMD cpus (found by Matt Dillon)

* Official reference for errata 721:

    http://support.amd.com/us/Processor_TechDocs/41322_10h_Rev_Gd.pdf

* This is the cpu bug found by Matthew Dillon, present on all K10 cores
  (so far).  AMD has indicated that the bug is not present on Bulldozer
  cpus.

* The MSR workaround is now reported and programmed during kernel boot.

* No discernable difference in performance was detected running GCC
  with and without the MSR adjustment.

* Verified to have fixed our test case w/GCC.  It no longer seg-faults.