Expose target libdir information via print command.

With custom libdir it is required to have an access to library
placement.

src/librustc_driver/lib.rs

@@ -676,6 +676,10 @@ impl RustcDefaultCalls {
                     println!("{}", targets.join("\n"));
                 }
                 Sysroot => println!("{}", sess.sysroot.display()),
+                TargetLibdir => println!(
+                    "{}",
+                    sess.target_tlib_path.as_ref().unwrap_or(&sess.host_tlib_path).dir.display()
+                ),
                 TargetSpec => println!("{}", sess.target.target.to_json().pretty()),
                 FileNames | CrateName => {
                     let input = input.unwrap_or_else(|| {

src/librustc_session/config.rs

@@ -391,6 +391,7 @@ impl ExternEntry {
 pub enum PrintRequest {
     FileNames,
     Sysroot,
+    TargetLibdir,
     CrateName,
     Cfg,
     TargetList,
@@ -1344,6 +1345,7 @@ fn collect_print_requests(
         "crate-name" => PrintRequest::CrateName,
         "file-names" => PrintRequest::FileNames,
         "sysroot" => PrintRequest::Sysroot,
+        "target-libdir" => PrintRequest::TargetLibdir,
         "cfg" => PrintRequest::Cfg,
         "target-list" => PrintRequest::TargetList,
         "target-cpus" => PrintRequest::TargetCPUs,