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/session/config.rs

@@ -163,6 +163,7 @@ pub enum PrintRequest {
     FileNames,
     Sysroot,
     CrateName,
+    Cfg,
 }
 
 pub enum Input {
@@ -1105,6 +1106,7 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
             "crate-name" => PrintRequest::CrateName,
             "file-names" => PrintRequest::FileNames,
             "sysroot" => PrintRequest::Sysroot,
+            "cfg" => PrintRequest::Cfg,
             req => {
                 early_error(error_format, &format!("unknown print request `{}`", req))
             }