x86: support newer Intel CPU models

x86: support newer Intel CPU models

This just follows what the January 2015 edition of the SDM documents,
with additional clarification from Intel:
- Broadwell models 0x4f and 0x56 don't cross-reference other tables,
  but should be treated like other Boradwell (0x3d),
- Xeon Phi model 0x57 lists LASTBRANCH_TOS but not where the actual
  stack is. Being told it's Silvermont based, attach it there.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>