x86, mce: mce.h cleanup

Reorder definitions.

 - static inline dummy mcheck_init() for !CONFIG_X86_MCE
 - gather defs for exception, threshold handler

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