x86: mce: Move code in mce.c

Now that the X86_OLD_MCE ifdefs are gone move some code that
used to be outside the big ifdef to a more natural place
near its user.

No code change.

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