Add the ability to have a 'fallback' search for memory ranges.

These set of ranges will be looked at if a standard memory
range isn't found, and won't be installed in the cache.
Use this to implement the memory behaviour of the PCI hole on
x86 systems, where writes are ignored and reads always return -1.
This allows breakpoints to be set when issuing a 'boot -d', which
has the side effect of accessing the PCI hole when changing the
PTE protection on kernel code, since the pmap layer hasn't been
initialized (a bug, but present in existing FreeBSD releases so
has to be handled).

Reviewed by:	neel
Obtained from:	NetApp

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