x86/mce: Add a wrapper around mce_log() for injection

Will be used by an injector module in a following patch.

Additionally, add a missing module export reported by 0-DAY
kernel test.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/1439396985-12812-13-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>