The `ci/run-docker.sh` utility script adds a `--volume` argument to the
`docker` command to mount the Cargo home directory of the host machine
into the container at `/cargo`.

Prior to this patch the host's Cargo home directory is assumed to be the
`dirname` of the `dirname` of the `cargo` command's path. That works in
most cases where the host machine installed rust with vanilla
`rustup`. It may fail if the host machine used a different method.

For example if the host machine used the Archlinux rustup package[0]
then `cargo` is installed to `/usr/bin/cargo` and the `run-docker.sh`
script incorrectly mounts `/usr/` to the `/cargo` directory of the test
container.

This patch allows specifying an explicit `CARGO_HOME` to the
`ci/run-docker.sh` script so that users with a non-standard cargo dir
can use the utility without modification. By default if no `CARGO_HOME`
is set then the legacy behaviour is used and `CARGO_HOME` defaults to
the `dirname` of the `dirname` of the `cargo` command is used.

[0]: https://wiki.archlinux.org/index.php/rust#Arch_Linux_package

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.