amd64: introduce minidump version 2

After KVA space was increased to 512GB on amd64 it became impractical
to use PTEs as entries in the minidump map of dumped pages, because size
of that map alone would already be 1GB.
Instead, we now use PDEs as page map entries and employ two stage lookup
in libkvm: virtual address -> PDE -> PTE -> physical address.  PTEs are
now dumped as regular pages.  Fixed page map size now is 2MB.

libkvm keeps support for accessing amd64 minidumps of version 1.
Support for 1GB pages is added.

Many thanks to Alan Cox for his guidance, numerous reviews, suggestions,
enhancments and corrections.

Reviewed by:	alc [kernel part]
MFC after:	15 days

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