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.

MFC after: 1 month