Add amd64-specific ddb command 'show phys2dmap', which calculates the address in the direct map corresponding to the given physical address.
Add amd64-specific ddb command 'show phys2dmap', which calculates the
address in the direct map corresponding to the given physical address.

Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

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