Improve --help performance for x.py

Since compiling the bootstrap command doesn't require any submodules,
we can skip updating submodules when a --help command is passed in.
On my machine, this saves 1 minute if the submodules are already
downloaded, and 10 minutes if run on a clean repo.

This commit also adds a message before compiling/downloading anything
when a --help command is passed in, to tell the user WHY --help
takes so long to complete. It also points the user to the bootstrap
README.md for faster help.

Finally, this fixes one warning message that still referenced using
make instead of x.py, even though x.py is now the standard way of
building rust.

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