Rollup merge of #137170 - ferrocene:pa-target-jemalloc, r=Kobzol

Allow configuring jemalloc per target

In Ferrocene we're trying to switch from `./configure` to a predefined `config.toml` file. One of the limitations of doing that is the `rust.jemalloc` configuration option, which we need to conditionally disable based on the target. This PR adds a `target.$tuple.jemalloc` option to override `rust.jemalloc` to make that possible.

Author: matthiaskrgr