rustc: Implement a new --print cfg flag

This commit is an implementation of the new compiler flags required by [RFC
1361][rfc]. This specifically adds a new `cfg` option to the `--print` flag to
the compiler. This new directive will print the defined `#[cfg]` directives by
the compiler for the target in question.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1361-cargo-cfg-dependencies.md

src/librustc_driver/lib.rs

@@ -518,6 +518,25 @@ impl RustcDefaultCalls {
                                       .to_string_lossy());
                     }
                 }
+                PrintRequest::Cfg => {
+                    for cfg in config::build_configuration(sess) {
+                        match cfg.node {
+                            ast::MetaWord(ref word) => println!("{}", word),
+                            ast::MetaNameValue(ref name, ref value) => {
+                                println!("{}=\"{}\"", name, match value.node {
+                                    ast::LitStr(ref s, _) => s,
+                                    _ => continue,
+                                });
+                            }
+                            // Right now there are not and should not be any
+                            // MetaList items in the configuration returned by
+                            // `build_configuration`.
+                            ast::MetaList(..) => {
+                                panic!("MetaList encountered in default cfg")
+                            }
+                        }
+                    }
+                }
             }
         }
         return Compilation::Stop;