GH-47588: [C++] Bump mimalloc version to 3.0.9

[pitrou]

Rationale for this change

According to microsoft/mimalloc#1073 , mimalloc v3 is preferred over v2 for production usage.

What changes are included in this PR?

Bump to mimalloc 3.0.9, which is the latest mimalloc v3 release as of this writing.

Are these changes tested?

Yes, by existing tests and CI.

Are there any user-facing changes?

Hopefully not, besides a potential reduction in memory usage due to improvements in mimalloc v3.

• GitHub Issue: [C++] Bump mimalloc version #47588

[pitrou]

@github-actions crossbow submit -g cpp

[pitrou]

@github-actions crossbow submit -g cpp

[pitrou]

@github-actions crossbow submit -g cpp

[pitrou]

@ursabot please benchmark

[voltrondatabot]

Benchmark runs are scheduled for commit 709942b. Watch https://buildkite.com/apache-arrow and https://conbench.ursa.dev for updates. A comment will be posted here when the runs are complete.

[conbench-apache-arrow]

Thanks for your patience. Conbench analyzed the 4 benchmarking runs that have been run so far on PR commit 709942b.

There were 80 benchmark results indicating a performance regression:

• Pull Request Run on amd64-c6a-4xlarge-linux at 2025-09-18 15:51:49Z
  • CopyBitmapWithoutOffset (C++) with params=8192, source=cpp-micro, suite=arrow-bit-util-benchmark
  • ReadUncachedFile (C++) with params=num_cols:64/is_partial:1/real_time, source=cpp-micro, suite=arrow-ipc-read-write-benchmark
• and 78 more (see the report linked below)

The full Conbench report has more details.

[pitrou]

@github-actions crossbow submit -g cpp

[github-actions]

Revision: 57aecbf

Submitted crossbow builds: ursacomputing/crossbow @ actions-35dfb14f79

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-cuda-cpp-ubuntu-22.04-cuda-11.7.1
test-debian-12-cpp-amd64
test-debian-12-cpp-i386
test-fedora-42-cpp
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