Auto merge of #55238 - alexcrichton:rm-jemalloc, r=estebank

Remove the `alloc_jemalloc` crate

This commit removes the `alloc_jemalloc` crate from the standard library and all related configuration. We will no longer be shipping this unstable crate. Rationale for this is provided on https://github.com/rust-lang/rust/issues/36963 and the many linked issues, but I can inline rationale here if desired!

We currently rely on jemalloc for increased perf in the Rust compiler, however. [This perf run shows](https://perf.rust-lang.org/compare.html?start=74ff7dcb1388e60a613cd6050bcd372a3cc4998b&end=7e7928dc0340d79b404e93f0c79eb4b946c1d669&stat=wall-time) that if we switch to glibc 2.23's allocator that it's slower than jemalloc across many benchmarks. [This perf run, however](https://perf.rust-lang.org/compare.html?start=22cc2ae8057d14e980b7c784e1eb2eee26b59e7d&end=10c95ccfa7a7adc12f4e608621ca29f9b98eed29), shows that if we use `jemalloc-sys` from crates.io then rustc actually gets faster across all benchmarks! (presumably because it has a more recent version of jemalloc than our submodule).

As a result, it's expected that this doesn't regress any code (as it's just removing an unstable crate) and it should actually improve rustc performance because it updates jemalloc.

Closes #36963

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