The wasm targets don't support gdb anyway so there's no need for this
section there.

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.

Suggest to add lifetime constraint at explicit ouput of functions

Closes rust-lang#62097

Add a proc-macro to derive HashStable in librustc dependencies

A second proc-macro is added to derive HashStable for crates librustc depends on.
This proc-macro HashStable_Generic (to bikeshed) allows to decouple code and some librustc's boilerplate.

Not everything is migrated, because `Span` and `TokenKind` require to be placed inside librustc.
Types using them stay there too.

Split out of rust-lang#66279
r? @Zoxc

Cleanup Miri SIMD intrinsics

r? @oli-obk @eddyb Cc @gnzlbg

Disable gdb pretty printer global section on wasm targets

The wasm targets don't support gdb anyway so there's no need for this
section there.

Point at type in `let` assignment on type errors

Fix rust-lang#61067.

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