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GH-44976: [C++] Enable mimalloc by default, disable jemalloc by default and more (#44951)

### What changes are included in this PR?

1) Disable jemalloc by default
2) Enable mimalloc by default
3) Some files in `arrow/compute` are only required when ARROW_COMPUTE is enabled, so we needn't compile them by default

### Are these changes tested?

Yes, by existing CI tests.

### Are there any user-facing changes?

Yes, but nothing compatibility-breaking.
* GitHub Issue: #44976

Authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>