x86/cpu: Add platform ID to CPU info structure

x86/cpu: Add platform ID to CPU info structure

The end goal here is to be able to do x86_match_cpu() and match on a
specific platform ID. While it would be possible to stash this ID
off somewhere or read it dynamically, that approaches would not be
consistent with the other fields which can be matched.

Read the platform ID and store it in cpuinfo_x86.

There are lots of sites to set this new field. Place it near
the place c->microcode is established since the platform ID is
so closely intertwined with microcode updates.

Note: This should not grow the size of 'struct cpuinfo_x86' in
practice since the u8 fits next to another u8 in the structure.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://patch.msgid.link/20260304181020.8D518228@davehans-spike.ostc.intel.com