Replace jemalloc bootstrap options with override-allocator

[Zoxc]

I want to make use of mimalloc on Windows so this renames jemalloc bootstrap options to override-allocator so we can reuse the same flag for this purpose.

[rustbot]

The Miri subtree was changed

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Warning

If you are changing how CI LLVM is built or linked, make sure to bump
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This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR modifies bootstrap.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

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[Kobzol]

Just renaming this doesn't make sense on its own; the config and the feature handling would have to be split. override-allocator would take the allocator to use, so it would become a string option, instead of a bool, but then we should still keep the individual features for the different allocators, i.e. jemalloc, mimalloc, etc.

[Zoxc]

The idea was to not expose control over which allocator gets used, so you couldn't specify mimalloc on Linux for example. Do we want to support more than one (on a specific target)?

[jieyouxu]

@rustbot reroll

[Zoxc]

We'll want different approaches on Linux, macOS and Windows to override the C / Rust allocators. Having a single Cargo feature seems like it will result in less cfg! soup than having multiple, but a feature per allocator is possible too.

I'd like some feedback on the direction to take here so I can start working on macOS and Windows PRs on top of this.

[Kobzol]

I feel like there are specific things that we want to do per the selected allocator (JEMALLOC_SYS_WITH_LG_PAGE), and also that we will want to repeatedly run benchmarks with jemalloc/mimalloc/etc. on the various OSes, to periodically evaluate their perf. So I'd go towards a feature per allocator. That will also make it easier for us to switch between the allocators on individual OSes more easily.

On the other hand, we will either have to try to build all of the allocators on CI to ensure that they still at least build, or risk that the unused OS/allocator combos might go stale and break.