Don't log machine check events left over from boot.  Too many BIOSes leave
bogus events in there.

This unfortunately also makes it impossible to log events that caused a
reboot.  For people with non broken BIOS there is mce=bootlog
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>