Recent build of vs2017-win2016 image does not have GNU find in path.
Instead, Windows built-in version was invoked, causing `Find GCC libraries`
Azure CI stage to fail.

i686-apple-darwin and other 32bit Apple targets ware demoted to tier-3 due
to lack of support from the vendor, and is no longer available from rustup
for nightly, shortly beta.

DockerOSX32 job was removed as it only tests on nightly. BuildChannelsOSX job
will continue testing on these targets on older compilers (stable and below).

This commit performs a number of updates for libc with the `wasm32-wasi`
target:

* Updates the `wasi-libc` repository commit used (previously known as
  `wasi-sysroot`)
* Updates the container to Ubuntu 19.10 which has Clang 9 packaged which
  is all we need.
* Avoids building `wasmtime` and instead downloads a precompiled binary.
* Updates bindings in `src/wasi.rs` to match the current upstream state.

We recently added a CDN in front of our CI mirrors as it's faster and
cheaper for us. This switches libc's CI to use it instead of accessing
the underlying bucket directly.

Should be a simple but effective fix for #1524. Assume `sort` command is available.

This reverts commit 89e8ae6a.

Previously mirrors were stored in the rust-lang-ci2 bucket, which is
meant to store temporary data (the CI artifacts). This switches the code
to fetch from the new mirrors bucket.

The old files won't be removed, but they won't be backed up either.