These no longer work now that Cargo changes the cwd of rustc while it's running.
Instead use an absolute path that's set by rustbuild.

This reverts commit ab018c76.

This also adds the `ToolBuild::is_ext_tool` field to replace the previous
`ToolBuild::expectation` field, to indicate whether a build-failure of
certain tool is essential.

This commit allocates a builder to running wasm32 tests on Travis. Not all test
suites pass right now so this is starting out with just the run-pass and the
libcore test suites. This'll hopefully give us a pretty broad set of coverage
for integration in rustc itself as well as a somewhat broad coverage of the llvm
backend itself through integration/unit tests.

This commit adds a new target to the compiler: wasm32-unknown-unknown. This
target is a reimagining of what it looks like to generate WebAssembly code from
Rust. Instead of using Emscripten which can bring with it a weighty runtime this
instead is a target which uses only the LLVM backend for WebAssembly and a
"custom linker" for now which will hopefully one day be direct calls to lld.

Notable features of this target include:

* There is zero runtime footprint. The target assumes nothing exists other than
  the wasm32 instruction set.
* There is zero toolchain footprint beyond adding the target. No custom linker
  is needed, rustc contains everything.
* Very small wasm modules can be generated directly from Rust code using this
  target.
* Most of the standard library is stubbed out to return an error, but anything
  related to allocation works (aka `HashMap`, `Vec`, etc).
* Naturally, any `#[no_std]` crate should be 100% compatible with this new
  target.

This target is currently somewhat janky due to how linking works. The "linking"
is currently unconditional whole program LTO (aka LLVM is being used as a
linker). Naturally that means compiling programs is pretty slow! Eventually
though this target should have a linker.

This target is also intended to be quite experimental. I'm hoping that this can
act as a catalyst for further experimentation in Rust with WebAssembly. Breaking
changes are very likely to land to this target, so it's not recommended to rely
on it in any critical capacity yet. We'll let you know when it's "production
ready".

---

Currently testing-wise this target is looking pretty good but isn't complete.
I've got almost the entire `run-pass` test suite working with this target (lots
of tests ignored, but many passing as well). The `core` test suite is still
getting LLVM bugs fixed to get that working and will take some time. Relatively
simple programs all seem to work though!

---

It's worth nothing that you may not immediately see the "smallest possible wasm
module" for the input you feed to rustc. For various reasons it's very difficult
to get rid of the final "bloat" in vanilla rustc (again, a real linker should
fix all this). For now what you'll have to do is:

    cargo install --git https://github.com/alexcrichton/wasm-gc
    wasm-gc foo.wasm bar.wasm

And then `bar.wasm` should be the smallest we can get it!

---

In any case for now I'd love feedback on this, particularly on the various
integration points if you've got better ideas of how to approach them!

This makes it mandatory for other steps to have to handle the potential
failure instead of failing in an odd way later down the road.

The RLS currently is rebuilt every time you test it because the `OPENSSL_DIR`
env var is changing, which is in turn caused by an accidental omission of
`prepare_tool_cargo` when testing the RLS.

Previously it would use the librustc output directory which would cause rustdoc
to get entirely recompiled, whereas the intention is that it uses the
already-compiled artifacts from building rustdoc itself, using the tool output
directory

Doc tests are temporarily disabled until next release cycle, since
current beta Cargo errors on them. Upgrade should be smooth as the
relevant tests are already fixed in this commit.

Like #43008 (f6689991), but _much more aggressive_.

This introduces a slight change in behavior, where we unilaterally
respect the --host and --target parameters passed for all sanity
checking and runtime configuration.

This commit leverages a relatively new feature in Cargo to execute
cross-compiled tests, the `target.$target.runner` configuration. We configure it
through environment variables in rustbuild and this avoids the need for us to
locate and run tests after-the-fact, instead relying on Cargo to do all that
execution for us.

For most tests, rustdoc isn't needed, so avoid building it.

Rustdoc is no longer compiled in every stage, alongside rustc, instead
it is only compiled when requested, and generally only for the last
stage.

This commit bumps the master branch's version to 1.21.0 and also updates the
bootstrap compiler from the freshly minted beta release.