Previously we would only backpatch overflowed address calculations
if the overflow was 0x1000 or less. Now we can handle the full 2 GiB
of overflow in both directions.
I'm also making equivalent changes to JitArm64's code. This isn't because
it needs it – JitArm64 address calculations should never overflow – but
because I wanted to get rid of the 0x100001000 inherited from Jit64 that
makes even less sense for JitArm64 than for Jit64.