Looks like Travis [has announced][blog] that our current `xcode8.2` image is
being deprecated and the recommended Xcode 7 image is `xcode7.3`. This updates
us to these ahead of time to make sure we can shake out any bugs, if any.

[blog]: https://blog.travis-ci.com/2017-10-16-a-new-default-os-x-image-is-coming

This commit disables debug assertions for OSX in an effort to improve cycle time
on OSX. It looks like #44610 didn't shave off quite as much time as desired so
let's see how much this helps.

Moving buckets from us-east-1 to us-west-1 because us-west-1 is where
rust-central-station itself runs and in general is where we have all our other
buckets.

Our OSX builders are routinely and significantly over hour 2 hour "soft limit"
for testing PRs. I *think* that a big portion of this time comes from the fact
that LLVM and debug assertions are enabled. In an effort to speed up these
builders and reduce cycle time this commit disables LLVM assertions on OSX for
all builders.

My thinking is that we'll let this bake for a bit after merged to see what the
effect is on timing on Travis. If it doesn't actually help much we can turn them
back on, and if it doesn't help enough we can disable Rust debug assertions as
well.

Most of the other rust-lang buckets are in us-west-1 and I think the original
bucket was just accidentally created in the us-east-1 region. Let's consolidate
by moving it to the same location as the rest of our buckets.

Haven't done this in awhile so seems like a good idea!

Travis recently pushed an update to the Linux environments, namely the kernels
that we're running on. This in turn caused some of the sanitizer tests we run to
fail. We also apparently weren't the first to hit these failures! Detailed in
google/sanitizers#837 these tests were failing due to a specific commit in the
kernel which has since been backed out, but for now work around the buggy kernel
that's deployed on Travis and eventually we should be able to remove these
flags.

Travis is in the process of [rolling out an update][update] but looks like our
tests are breaking, let's temporarily roll back to get the queue moving again.

[update]: https://blog.travis-ci.com/2017-08-29-trusty-image-updates

Reorder the job matrix to take advantage of the order how Travis CI starts
them in rust-lang/rust. Plus other refactoring of `.travis.yml`.

1. Move the `$ALLOW_PR` image to the top, so users' PRs will start testing
   immediately. Previously the `$ALLOW_PR` image starts 6 minutes after the
   build was scheduled.

2. Move the slow macOS images near the top, so they share more time with
   the rest of the faster Linux builds, which should shorten total test
   time (actually not much, about 7 minutes at most if this change does
   work).

3. Merged the `install` section of both Linux and macOS to make the `env:`
   section a bit shorter, and enable change 4 below.

4. Do not download or install anything if `$SKIP_BUILD == true`, which
   further reduces chance of spurious failure in the PR-CI stage (avoid the
   red cross appearing even if CI passed).

Use the `-f` flag to indicate that, for example, a 500 response code is to be
considered a failure, triggering the normal retry logic. Also ignore errors
where we check the date from google.com, as a failure there shouldn't fail the
build.

In theory, this will give us more disk space to work with and unblock the queue.

This hopefully fixes #42967 and #43085.

- rename emscripten docker image to asmjs
- create wasm32 docker image

This saves us builders for more helpful purposes.

There is no `minimal` language. Due to travis-ci/travis-ci#4895, it will
fallback to `ruby`, which certainly isn't what we want. `generic` is an
undocumented (travis-ci/docs-travis-ci-com#910) language that serves the
desired purpose.

* Bring back colors on Travis, which was disabled since #39036.
  Append --color=always to cargo when running in CI environment.
* Removed `set -x` in the shell scripts. The `retry` function already
  prints which command it is running, add `-x` just add noise to the
  output.
* Support travis_fold/travis_time. Matching pairs of these allow Travis CI
  to collapse the output in between. This greatly cut down the unnecessary
  "successful" output one need to scroll through before finding the failed
  statement.

This commit updates the sccache binaries to fix a cache load failure seen
on #41926, fixed by mozilla/sccache#119

Wrap the installation on macOS with `travis_retry`.

MacOSX does not ship `7z` nor `xz`. Let's use `xz`, just like on the
other *nix systems.

Pulls in mozilla/sccache@ef0d77543 to fix #40240 again after the builds included
in #41447 forgot to include the mio fixed included in #41076.

Closes #40240

Now that the final bug fixes have been merged into sccache we can start
leveraging sccache on the MSVC builders on AppVeyor instead of relying on the
ad-hoc caching strategy of trigger files and whatnot.

This build is no longer a forked version with temporary bugfixes, everything
should be upstreamed!

ASan and TSan are supported on macOS, and this commit enables their
support.

The sanitizers are always built as *.dylib on Apple platforms, so they
cannot be statically linked into the corresponding `rustc_?san.rlib`. The
dylibs are directly copied to `lib/rustlib/x86_64-apple-darwin/lib/`
instead.

Note, although Xcode also ships with their own copies of ASan/TSan dylibs,
we cannot use them due to version mismatch.

There is a caveat: the sanitizer libraries are linked as @rpath, so the
user needs to additionally pass `-C rpath`:

    rustc -Z sanitizer=address -C rpath file.rs
                               ^~~~~~~~

Otherwise there will be a runtime error:

    dyld: Library not loaded: @rpath/libclang_rt.asan_osx_dynamic.dylib
      Referenced from: /path/to/executable
      Reason: image not found
    Abort trap: 6

The next commit includes a temporary change in compiler to force the linker
to emit a usable @rpath.

I've tracked down what I believe is the last spurious sccache failure on #40240
to behavior in mio (carllerche/mio#583), and this commit updates the binaries to
a version which has that fix incorporated.