rustc: Don't use relative paths for extended errors

These no longer work now that Cargo changes the cwd of rustc while it's running.
Instead use an absolute path that's set by rustbuild.

src/libsyntax/lib.rs

@@ -151,4 +151,5 @@ pub mod ext {
 #[cfg(test)]
 mod test_snippet;
 
+#[cfg(not(stage0))] // remove after the next snapshot
 __build_diagnostic_array! { libsyntax, DIAGNOSTICS }