This allows argv[0] to be overridden on the executable's command-line. This also makes the program
executed independent of argv[0].

Does Fuchsia have the same semantics?

Addresses: rust-lang#66510

Some environment variables (like DEPLOY or DEPLOY_ALT for dist builders,
or IMAGE on Linux builders) are set on a lot of builders, and whether
they should be present or not can be detected automatically based on the
builder name and the platform.

This commit simplifies the CI configuration by automatically setting
those environment variables.

Add unix::process::CommandExt::arg0

This allows argv[0] to be overridden on the executable's command-line. This also makes the program
executed independent of argv[0].

Does Fuchsia have the same semantics? I'm assuming so.

Addresses: rust-lang#66510

GitHub Actions: preparations, part 1

This PR adds the first batch of commits in preparation for GitHub Actions:

* Added GitHub Actions support in `src/ci/shared.sh` and bootstrap.
* Addded a `setup-environment.sh` script which guesses and sets the `DEPLOY`, `DEPLOY_ALT` and `IMAGE` environment variables automatically, to reduce the verbosity of the CI configuration.

This PR does **not** yet add any builders on GitHub Actions.

r? @alexcrichton

…=davidtwco

Drive-by cleanup in region naming

Add some comments to panic runtime

…acrum

tidy: Remove unused import

It produces annoying warnings on `x.py` runs.

…entril

Move ErrorReported to rustc_errors

The new location is more consistent with what this type is for, though we don't remove it from the old location (via a re-export) to avoid changing the dozens of use sites (~139 at this time).