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.

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.

* Update Android NDK to version r19c
* Update Android API versions to:
  * API 24 on arm and aarch64 targets
  * API 28 on x86 and x86_64 targets
* Unified headers were removed in NDK 16
* Refactor the NDK and SDK installation scripts
* OpenJDK version 8 must be kept: it appears that
  the Android tools do not work with more modern OpenJDK
  versions.

This reverts commit 1a1b170a.

This reverts commit 7b171fb1.

This contains several fixes, including the FD_ISSET signature change
that the previous patch in this PR needs.