x86, mce: synchronize core after machine check handling

The example code in the IA32 SDM recommends to synchronize the CPU
after machine check handling. So do that here.

[ Impact: Spec compliance ]

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>