refactor(opt-dist): use rustc-perf from rustc checkout

This replaces the hardcoded rustc-perf commit and ad-hoc downloading and
unpacking of its zipped source with defaulting to use the new rustc-perf
submodule.

While it would be nice to make `opt-dist` able to initialize the
submodule automatically when pointing to a Rust checkout _other_ than
the one opt-dist was built in, that would require a bigger refactor that
moved `update_submodule`, from bootstrap, into build_helper.

Regardless, I imagine it must be quite rare to use `opt-dist` with a
checkout that is neither from a rust-src tarball (which will contain the
submodule), nor the checkout opt-dist itself was built (bootstrap will
update the submodule when opt-dist is built).

Author: lovesegfault