Fix compiling some rustc crates to wasm

I was dabbling recently seeing what it would take to compile `rustfmt` to the
`wasm32-unknown-unknown` target and it turns out not much effort is needed!
Currently `rustfmt` depends on a few rustc crates published to crates.io, so
this commit touches up those crates to compile for wasm themselves. Notably:

* The `rustc_data_structures` crate's `flock` implementation is stubbed out to
  unconditionally return errors on unsupported platforms.
* The `rustc_errors` crate is extended to not do any locking for all non-windows
  platforms.

In both of these cases if we port the compiler to new platforms the
functionality isn't critical but will be discovered over time as it comes up, so
this hopefully doesn't make it too too hard to compile to new platforms!

Author: alexcrichton