tracing: Add the ::ppin field to the mce_record tracepoint

Machine Check Error information from 'struct mce' is exposed to userspace
through the mce_record tracepoint.

Currently, however, the PPIN (Protected Processor Inventory Number) field
of 'struct mce' is not exposed.

Add a PPIN field to the tracepoint as it provides a unique identifier for
the system (or socket in case of multi-socket systems) on which the MCE
has been received.

Also, add a comment explaining the kind of information that can be and
should be added to the tracepoint.

Signed-off-by: Avadhut Naik <avadhut.naik@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20240401171455.1737976-2-avadhut.naik@amd.com