Enhance debug output for exception handlers

[joschock]

This is a request for enhancements to the debug output included in x86 CPU exception handler debug prints.

Two enhancements are requested:

1. For MachineCheck exceptions, add prints for the architectural Machine Check MSRs (MCi_STATUS, MCi_ADDR, etc.)
2. Add a dump of the pre-exception stack for exceptions where the stack pointer is valid.