Auto merge of #43630 - Mark-Simulacrum:rustbuild-cleanups, r=alexcrichton

Rustbuild cleanups/fixes and improvements

Each commit is a standalone change, and can/should be reviewed separately.

This adds two new functionalities:

 - `--target` and `--host` can be passed without changing config.toml, and we'll respect the users' wishes, instead of requiring that all possible targets are passed.
   - Note that this means that `./x.py clean` won't be quite as wide-spread as before, since it limits itself to the configured hosts, not all hosts. This could be considered a feature as well.
 - `ignore-git` field in `config.toml` which tells Rustbuild to not attempt to load git hashes from `.git`.

This is a precursor to eventual further simplification of the configuration system, but I want to get this merged first so that later work can be made in individual PRs.

r? @alexcrichton

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