Auto merge of #1342 - sunfishcode:master, r=gnzlbg

Update to the latest wasi-sysroot.

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