Simplify jemalloc setup

[madsmtm]

In the past, #[used] had to appear in the top-level crate to have a consistent effect on the linker. This has been fixed a while ago for ELF with the introduction of the symbols.o file in #95604, and more recently for Mach-O in #133832, which means that libraries can now implement the required workarounds themselves. This allows moving these #[used] declarations out of our main.rs.

Specifically, I have moved them into tikv-jemalloc-sys where they belong in tikv/jemallocator#109 and done the same for mimalloc in purpleprotocol/mimalloc_rust#146 (in case we want to experiment with switching to that one day).

Test with:

./x build library src/tools/rustdoc src/tools/clippy --set rust.jemalloc=true

# macOS
lldb -- ./build/host/stage1/bin/rustc -vV
(lldb) b _rjem_je_zone_register
(lldb) run
# Should breakpoint, this means that the allocator was properly linked

# Linux
lldb -- ./build/host/stage1/bin/rustc -vV
(lldb) b malloc
(lldb) run
# Should breakpoint, inspect that the `malloc` symbol comes from the `rustc` binary and not from `libc`

try-job: aarch64-gnu
try-job: dist-aarch64-linux
try-job: dist-x86_64-musl
try-job: dist-x86_64-apple
try-job: dist-aarch64-apple

[madsmtm]

tikv-jemalloc-sys v0.6.1 has been released with the mentioned PR, so this is now ready. I'm less certain about the Linux parts here, so please double-check that.

Also, in the second commit, I fixed librustdoc and clippy building with --features jemalloc / ./x build --set rust.jemalloc=true, unclear to me how this wasn't caught by CI? Are these built differently there?

@rustbot ready
r? compiler
@bors try

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

The Miri subtree was changed

cc @rust-lang/miri

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 8d39231 (8d39231d22563844d4088a7af200b6885c749c85, parent: 79966ae420f38c5861d177356a3446023c090d6d)

[bors]

☔ The latest upstream changes (presumably #148412) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.