powerpc: Add a CPU-custom machine check handler

Summary:
Some machine checks are process-recoverable, others are not. Let a
CPU-specific handler decide what to do.

This works around a machine check error hit while building www/firefox
and mail/thunderbird, which would otherwise cause the build to fail.

More work is needed to handle all possible machine check conditions, but
this is sufficient to unblock some ports building.

Differential Revision: https://reviews.freebsd.org/D23731