Rollup merge of #158775 - valentynkit:bootstrap-readable-rustflags, r=Mark-Simulacrum

bootstrap: only encode RUSTFLAGS when a flag contains a space

Fixes rust-lang/rust#158749

Since rust-lang/rust#158073, bootstrap passes rustc and rustdoc flags to cargo through `CARGO_ENCODED_RUSTFLAGS` / `CARGO_ENCODED_RUSTDOCFLAGS`, whose values are `\x1f`-separated. When a command fails and gets debug-printed, that separator shows up as `\u{1f}`, e.g.:

```
CARGO_ENCODED_RUSTDOCFLAGS="--cfg=windows_raw_dylib\u{1f}-Csymbol-mangling-version=v0\u{1f}..."
```

which isn't a valid escape in bash/zsh, so the printed command can't be copy-pasted.

The encoded form is only needed when a flag value contains a space (the case rust-lang/rust#158073 fixed, e.g. an `-L` path from `llvm-config --libdir` under a directory whose name has a space). Cargo splits `RUSTFLAGS` on spaces, so when no flag contains one the plain, space-separated `RUSTFLAGS`/`RUSTDOCFLAGS` form is equivalent and stays readable. This uses the plain form in that case and only falls back to the encoded form when a flag actually contains a space.

It also clears any inherited `CARGO_ENCODED_RUSTFLAGS`/`CARGO_ENCODED_RUSTDOCFLAGS`, since cargo prefers the encoded variable over the plain one and the plain form would otherwise be ignored.

r? @Mark-Simulacrum