Add --toolchain option for test command

[GuillaumeGomez]

No description provided.

[antoyo]

This does not appear to work.
When running:

./y.sh test --toolchain cg_gcc-stage1

I get a bunch of errors like:

error[E0599]: no method named `add_used_global` found for reference `&CodegenCx<'gcc, 'tcx>` in the current scope
   --> src/consts.rs:167:14
    |
167 |         self.add_used_global(global);
    |              ^^^^^^^^^^^^^^^

as though it would compile (the local) cg_gcc itself with the (remote) toolchain.

[rustbot]

☔ The latest upstream changes (possibly #863) made this pull request unmergeable. Please resolve the merge conflicts.