MFC 308820,308821: Fixes for fatal page faults on x86.

308820:
Report page faults due to reserved bits in PTEs as a separate fault type.

Rather than reporting a page fault due to a bad PTE as a protection
violation with the "rsv" flag, treat these faults as a separate type of
fault altogether.

308821:
MFamd64: Various fatal page fault fixes.

• If a page fault is triggered due to reserved bits in a PTE, treat it as a fatal fault and panic.
• If PG_NX is in use, report whether a fatal page fault is due to an instruction fetch or a data access.
• If a fatal page fault is due to reserved bits in a PTE, report that as the page fault type rather than a protection violation.