Enable jemalloc on MacOS

[Velfi]

This PR enables the jemalloc allocator on MacOS by default. Previously, this was only done for Linux. We're making this change because it will make memory profiling easier than it currently is.

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  • Manual tests, as necessary

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[apollo-librarian]

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Build ID: bb012789bb0c53acf458d749
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[BrynCooke]

We still need to consider windows. Please can you verify that either jemalloc will work on windows or reintroduce the feature gate so that we only cover the right platforms.

Edit: I see you specified unix. Ignore this.