add(build_toolchains): New command to build cross and native target toolchains.

This replaces the cross-toolchain compile step in bootstrap_sdk and adds the
ability to build native toolchains using the cross toolchain. This is just
the first step towards actually providing the native toolchain in a container.
fix(common.sh): Stop collecting stats and uploading them to Google.

Just... no.. in so many ways. Kinda dumb it took me this long to delete
this little chunk of rather annoying code.