Generate the toolchain rules in a single separate package so users can use the :all target pattern when registering them

[rrbutani]

local_config_cc_toolchains does this (toolchains are defined here and here).

This is desirable because it makes it possible to just use @llvm_toolchain//toolchains:all (for example) when using this toolchain with a toolchain transition that sets --extra_toolchains rather than meticulously maintaining a select call that matches the toolchains that this repo emits for each platform/configuration.

We should also update the blurb in the readme that mentions using --extra_toolchains=... to match once we do this.

(this isn't high-priority; just something I ran into while trying to put together a proof of concept for #130)