Symlink /ports/cef/target to /target, to reduce compile time.

Currently, `mach build-cef` uses a completely separate target directory
than `mach build`, so everything gets built twice.

Now that we’ve upgraded Rust to a version that fixes
https://github.com/rust-lang/rust/issues/16496,
we can make this symlink to share target directories.

"git" dependencies builds are shared as expected, but "path" dependecies
still get rebuilt for some reason that might be related to
https://github.com/rust-lang/cargo/issues/497

@alexcrichton says this is unsupported behavior and we shouldn’t do it,
but it still is strictly better than what we have now as far as I can tell.