Auto merge of #123520 - cuviper:bootstrap-compiler-rustflags, r=Mark-Simulacrum

bootstrap: move all of rustc's flags to `rustc_cargo`

This ensures that `RUSTFLAGS` will be consistent between all modes of
building the compiler, so they won't trigger a rebuild by cargo. This
kind of fix was started in #119414 just for LTO flags, but it's
applicable to all kinds of flags that might be configured.

Author: bors