don't machine check when a PCI Master Abort is signalled. This can
happen, for instance, when configuration space for a device that isn't
present is examined. When this is detected, act like we normally would
when machine checks are posted while examining nonexistant devices.
Support for AXPpci CPUs,
Support for AlphaStation 600 CPUs,
new boot block structure, which requires an 'installboot'
program and works a lot like the NetBSD/sparc boot blocks.