x86, mce, cmci: avoid potential reentry of threshold interrupt

Impact: minor bugfix

The threshold handler on AMD (and soon on Intel) could be theoretically
reentered by the hardware. This could lead to corrupted events
because the machine check poll code assumes it is not reentered.

Move the APIC ACK to the end of the interrupt handler to let
the hardware avoid that.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>