Update wasi-libc to latest HEAD (#290)

* Update wasi-libc to latest HEAD

This pulls in a variety of changes, many of them related to enabling
threads in wasi-libc. There may be more changes necessary before
releasing a prelease version of wasi-sdk that supports the new
`wasm32-wasi-threads` target, but this should get the majority of them.

* review: incorporate WebAssembly/wasi-libc#324