Add intiial support for wasm32-unknown-wasi

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]: