rustbuild: Add sccache support
This commit adds support for sccache, a ccache-like compiler which works on MSVC
and stores results into an S3 bucket. This also switches over all Travis and
AppVeyor automation to using sccache to ensure a shared and unified cache over
time which can be shared across builders.
The support for sccache manifests as a new `--enable-sccache` option which
instructs us to configure LLVM differently to use a 'sccache' binary instead of
a 'ccache' binary. All docker images for Travis builds are updated to download
Mozilla's tooltool builds of sccache onto various containers and systems.
Additionally a new `rust-lang-ci-sccache` bucket is configured to hold all of
our ccache goodies.
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Note that this does not currently change Windows [due to previously written up issues](https://github.com/rust-lang/rust/issues/38119#issuecomment-266631585). Despite that, however, I was curious to get timings for the builds on Travis to see what ranges we're working with. As a result, this is a WIP PR I'm using to gauge build times and such.
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