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GH-44372: [C++] Fix unaligned load/store implementation for clang-18 (#44468)

### Rationale for this change

CLang 18 complains about undefined behavior when memcpy is called on a T-unaligned `T*` pointer.
This is due to this upstream change: https://github.com/llvm/llvm-project/pull/67766

### What changes are included in this PR?

Workaround by casting to `void*`.

### Are these changes tested?

By existing CI tests.

### Are there any user-facing changes?

No.

* GitHub Issue: #44372

Lead-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>