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
rust-lang/rust#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
rust-lang/cargo#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.

.gitignore

@@ -1,7 +1,6 @@
 /target
 /rust
 /cargo
-/ports/cef/target
 /ports/android/bin
 /ports/android/libs
 /ports/android/local.properties

ports/cef/target

@@ -0,0 +1 @@
+/home/simon/tmp/cargo-targets/servo