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 in 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.

I have done so for tikv-jemalloc-sys in tikv/jemallocator#109 (and for mimalloc in purpleprotocol/mimalloc_rust#146, in case we want to experiment with switching to that one day), so waiting for those to land before this is ready.
r? ghost
@rustbot blocked

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

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Caused by:
  Unable to update /jemallocator/jemalloc-sys

Caused by:
  failed to read `/jemallocator/jemalloc-sys/Cargo.toml`

Caused by:
  No such file or directory (os error 2)
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:28