aarch64: add shims for pairwise widening/wrapping addition

[folkertdev]

Combined with #4899 this is sufficient to run the zlib-rs crc32 tests.

With LLVM 23 I believe that these implementations should no longer be needed because the stdarch implementation can use portable intrinsics. Until then, we do need them, and the test coverage is nice regardless.

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[RalfJung]

Shouldn't this be strict_add? An overflow would lose data which sounds bad. (Probably also applies above.)

[folkertdev]

I don't think it matters because it's widening. I'll change it though

[RalfJung]

@rustbot author

[rustbot]

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[folkertdev]

@rustbot ready

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

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[RalfJung]

This looks great, thanks! Please squash the commits. You can squash manually if there are multiple independent commits you want to preserve, or use ./miri squash (make sure to pick a suitable commit message). Then write @rustbot ready after you force-pushed the squashed PR.

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