Core: Combine guest pages into host pages larger than 4K

Most systems that Dolphin runs on have a page size of 4 KiB, which
conveniently matches the page size of the GameCube and Wii. But there
are also systems that use larger page sizes, notably Apple CPUs with
16 KiB page sizes. To let us create host mappings on such systems, this
commit implements combining guest mappings into host page sized mappings
wherever possible.

For this to work for a given mapping, not only do four (in the case of
16 KiB) guest mappings have to exist adjacent to each other, but the
corresponding translated addresses also have to be adjacent, and the
lowest bits of the addresses have to match. When I tested a few games,
the following percentages of guest mappings met these criteria:

Spider-Man 2: 0%-12%
Rogue Squadron 2: 39%-42%
Rogue Squadron 3: 28%-41%

So while 16 KiB systems don't get as much of a performance improvement
as 4 KiB systems, they do still get some improvement.

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