Fix `x build library/std compiler/rustc`

Previously, this was broken because of improper caching:
1. `StepDescription::maybe_run` builds `Compile::Std`, which only built `std` and not `proc_macro`
1. `Std` calls `builder.ensure(StdLink)`
1. `Rustc` calls `ensure(Std)`, which builds all crates, including `proc_macro`
1. `Rustc` calls `ensure(StdLink)`. `ensure` would see that it had already been run and do nothing.  <-- bug is here
1. Cargo gives an error that `proc_macro` doesn't exist.

This fixes the caching by adding `crates` to `StdLink`, so it will get rerun if the crates that are
built change.  This also does the same for `RustcLink`; it doesn't matter in practice currently
because nothing uses it except `impl Step for Rustc`, but it will avoid bugs if we start using it in
the future (e.g. to build individual crates for rustfmt).

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