x86, mce: Fake panic support for MCE testing

If "fake panic" mode is turned on, just log panic message instead of
go real panic. This is used for testing only, so that the test suite
can check for the correct panic message and do regression testing for
MCE would go panic.

This patch is based on x86-tip.git/mce.

ChangeLog:

v5:

- Rebased on x86-tip.git/mce

v4:

- Move config file from sysfs to debugfs

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>