Define `O_NOCTTY`, `R_OK`, `POLLIN`, and related constants using the
current values from WASI libc.

This adds various WASI libc definitions to the Rust libc bindings that I
needed while porting some applications to WASI.

It also removes the `pause` binding since newer versions of WASI libc
have removed this function as well. (WASI currently has no syscall with
this functionality.)

These declarations are the same as those for other platforms.

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.

 - Rename `wasm32-unknown-wasi` to `wasm32-wasi`.
 - `__wasilibc_rmfileat` was renamed to `__wasilibc_unlinkat`
 - Add bindings for a few more functions and typedefs.

fd_set isn't automatically copyable. While it will be possible to fix
that, for now just remove these so that they don't block other changes.

dirent contains a flexible array member, so don't test its sizeof, don't
allow it to be copied, and don't represent it with an artificial size.

Also, locale_t is a typedef for a pointer to an opaque struct, so
represent it that way explicitly.

Added recently and will be used in libstd!

Now that wasi is in nightlies, we can run it on PRs!

This target is [being proposed][LINK] int he rust-lang/rust repository
and this is intended to get coupled with that proposal. The definitions
here all match the upstream reference-sysroot definitions and the
functions all match the reference sysroot as well. The linkage here is
described more in detail on the Rust PR itself, but in general it's
similar to musl.

Automatic verification has been implemented in the same manner as other
targets, and it's been used locally to develop this PR and catch errors
in the bindings already written (also to help match the evolving sysroot
of wasi). The verification isn't hooked up to CI yet though because
there is no wasi target distributed via rustup just yet, but once that's
done I'll file a follow-up PR to execute verification on CI.

[LINK]: