EXP: [C++] Bump mimalloc to 3.2.7

[pitrou]

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[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

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[pitrou]

@github-actions crossbow submit -g cpp

[pitrou]

@github-actions crossbow submit wheelcp314*

[pitrou]

@ursabot please benchmark

[pitrou]

Two CI failures are relevant, I've opened upstream issues:

• assertion failures on Debian i386
• crash on macOS if forking from non-main thread

[pitrou]

@ursabot please benchmark

[pitrou]

@github-actions crossbow submit -g cpp

[pitrou]

@github-actions crossbow submit wheelcp314*

[pitrou]

I've diagnosed the test freezes on Debian i386 and reported the issue.

[pitrou]

@github-actions crossbow submit -g r

[pitrou]

@github-actions crossbow submit -g cpp

[pitrou]

@github-actions crossbow submit -g packaging

[pitrou]

@github-actions crossbow submit -g r -g cpp

[github-actions]

Revision: aa3b3ff

Submitted crossbow builds: ursacomputing/crossbow @ actions-61a82dee27

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
r-binary-packages
r-recheck-most
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-debian-12-cpp-amd64
test-debian-12-cpp-i386
test-debian-experimental-cpp-gcc-15
test-fedora-42-cpp
test-r-alpine-linux-cran
test-r-arrow-backwards-compatibility
test-r-depsource-system
test-r-dev-duckdb
test-r-devdocs
test-r-extra-packages
test-r-gcc-11
test-r-gcc-12
test-r-install-local
test-r-install-local-minsizerel
test-r-linux-as-cran
test-r-linux-rchk
test-r-linux-sanitizers
test-r-linux-valgrind
test-r-m1-san
test-r-macos-as-cran
test-r-offline-maximal
test-r-ubuntu-22.04
test-r-versions
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-bundled
test-ubuntu-22.04-cpp-emscripten
test-ubuntu-22.04-cpp-no-threading
test-ubuntu-24.04-cpp
test-ubuntu-24.04-cpp-bundled-offline
test-ubuntu-24.04-cpp-gcc-13-bundled
test-ubuntu-24.04-cpp-gcc-14
test-ubuntu-24.04-cpp-minimal-with-formats
test-ubuntu-24.04-cpp-thread-sanitizer