x86, mce: Support specifying raise mode for software MCE injection

Raise mode include raising as exception or raising as poll, it is
specified via the mce.inject_flags field.

This can be used to specify raise mode of UCNA, which is UC error but
raised not as exception. And this can be used to test the filter code
of poll handler or exception handler too. For example, enforce a poll
raise mode for a fatal MCE.

ChangeLog:

v2:

- Re-base on latest x86-tip.git/mce3

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