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GH-44249: [C++] Unify simd header includings (#44250)



### Rationale for this change

#44249

### What changes are included in this PR?

Change all the system header inclusions to `simd.h`.

Also removed some duplicated defines.

### Are these changes tested?

Existing tests should be good.

### Are there any user-facing changes?

None.

* GitHub Issue: #44249

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>