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GH-43254: [C++] Always prefer mimalloc to jemalloc (#40875)

### Rationale for this change

As discussed [on the mailing-list](https://lists.apache.org/thread/dts9ggvkthczfpmd25wrz449mxod76o2), this PR switches the default memory pool to mimalloc for all platforms. This should have several desirable effects:

* less variability between platforms
* mimalloc generally has a nicer, more consistent API and is easier to work with (in particular, jemalloc's configuration scheme is slightly abtruse)
* potentially better performance, or at least not significantly worse, than the statu quo

### Are these changes tested?

Yes, by existing CI configurations.

### Are there any user-facing changes?

Behavior should not change. Performance characteristics of some user workloads might improve or regress, but this is something we cannot predict in advance.

* GitHub Issue: #43254

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>