Core: Allocate 2 GiB of guard pages below fastmem area

See the comment added by this commit. We were previously guarding against
overshooting in address calculations, but not against undershooting.
Perhaps someone assumed that the displacement of an x86 loadstore was
treated as unsigned?

Note: While the comment says we can undershoot by up to 2 GiB, in
practice Jit64 as it currently behaves won't actually undershoot by more
than 0x8000 if my analysis is correct. But address space is cheap, so
let's guard the full 2 GiB.

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