Rollup merge of rust-lang#94844 - bjorn3:rustbuild_cleanup, r=Mark-Simulacrum

Reduce rustbuild bloat caused by serde_derive

This reduces the size of the `.text` section from 10.1MiB (6.2MiB for just rustbuild code) to 9.3MiB (5.3MiB for just rustbuild code).

This also reduces compile time from ~6.1s for incr recompilation to ~5.6s.

r? `@Mark-Simulacrum`

Author: matthiaskrgr