x86/mce: Add mechanism to safely save information in MCE handler

Machine checks on Intel cpus interrupt execution on all cpus, regardless
of interrupt masking.  We have a need to save some data about the cause
of the machine check (physical address) in the machine check handler that
can be retrieved later to attempt recovery in a more flexible execution
state.

Signed-off-by: Tony Luck <tony.luck@intel.com>