Auto merge of #38708 - alexcrichton:add-distcheck, r=brson

Gate on distcheck on Travis

This commit adds a new entry to the Travis matrix to gate on distcheck, the illustrious test process that has historically taken *8 hours* to complete and also breaks all the time on nightly. By adding it to Travis we should hope to never see nightly breakage (like https://github.com/rust-lang/rust/issues/38690) because of this ever again!

"But wait, surely we can't wait 8 hours for all PRs!" you might be thinking, and you are indeed correct. The distcheck added here is much more optimized for speed than the old buildbot instances for a number of reasons:

* We're not building *two host compilers* beforehand. The current distcheck bot does a cross for i686 Linux and x86_64 Linux before it actually runs distcheck, building 6 compilers and LLVM twice. None of this is done in parallel as well (e.g. `-j1`). Not doing any of this work will be a huge win!
* We're using sccache to compile LLVM, so it should be much faster. Distcheck on the bots didn't cache LLVM well and rebuilt it every time.

All in all, this version of "distcheck" should be exactly like other matrix entries that run tests except that it's a *little* slower to start as it has to create the source tarball then rebuild the build system in the distcheck dir. Overall this should be well under the 2 hours that Android is currently taking anyway.

Closes https://github.com/rust-lang/rust/issues/38691

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