Add -Z cross-lang-lto flag in order to support linker-based LTO.

Author: michaelwoerister

src/librustc/session/config.rs

@@ -1304,6 +1304,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "tell the linker to strip debuginfo when building without debuginfo enabled."),
     share_generics: Option<bool> = (None, parse_opt_bool, [TRACKED],
           "make the current crate share its generic instantiations"),
+    cross_lang_lto: bool = (false, parse_bool, [TRACKED],
+          "generate build artifacts that are compatible with linker-based LTO."),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc_trans/back/write.rs

@@ -314,7 +314,8 @@ impl ModuleConfig {
         self.inline_threshold = sess.opts.cg.inline_threshold;
         self.obj_is_bitcode = sess.target.target.options.obj_is_bitcode;
         let embed_bitcode = sess.target.target.options.embed_bitcode ||
-            sess.opts.debugging_opts.embed_bitcode;
+                            sess.opts.debugging_opts.embed_bitcode ||
+                            sess.opts.debugging_opts.cross_lang_lto;
         if embed_bitcode {
             match sess.opts.optimize {
                 config::OptLevel::No |