x86, mce: enable machine checks in 64-bit defconfig

Impact: Low priority fix

The 32-bit defconfig already had it enabled. And it's a pretty
fundamental feature, so better enable it on 64 bits too.

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