Auto merge of #148925 - madsmtm:jemalloc-perf, r=Kobzol

Simplify `jemalloc` setup (without perf regression)

Reland rust-lang/rust#146627 after fixing [the performance regression](rust-lang/rust#148851 (comment)) that caused it to be reverted in rust-lang/rust#148896.

This avoids 65f0b7a (second commit in the initial PR), and adds a comment explaining why `extern crate` is needed here instead of `use` (we need to load `tikv_jemalloc_sys` from the sysroot because of rust-lang/cc-rs#1613).

r? Kobzol

Author: bors