x86: reorganize code that deals with unexpected NMI-s

Expected NMI-s are those than are either generated by the software (such
as a CPU sending NMI to other CPU) or generated by the hardware after
the software configured it to do so (such as NMI-s on PMC events).

Some unexpected NMI-s can be caused by hardware failures and it is
possible to inquire the hardware about them (somewhat like MCA but much
more primitive) using an EISA mechanism. In some cases the origin of
the NMI can remain truly unknown.

This commit should not change any functionality. It just reorganizes
the code, so that it is easier to extend with new checks for the origin
of the NMI. Also, it frees the code that has nothing to do with ISA
from DEV_ISA.

MFC after: 3 weeks