rustc_trans: Fix PGO generation linking on Linux by adding the relevant linker commands.

See the linked LLVM reviews for the clang counter-parts.

Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

Author: emilio

src/librustc_trans/back/link.rs

@@ -1085,6 +1085,20 @@ fn link_args(cmd: &mut Linker,
         cmd.build_static_executable();
     }
 
+    // If we're doing PGO generation stuff and on a GNU-like linker, use the
+    // "-u" flag to properly pull in the profiler runtime bits.
+    //
+    // This is because LLVM otherwise won't add the needed initialization for us
+    // on Linux (though the extra flag should be harmless if it does).
+    //
+    // See https://reviews.llvm.org/D14033 and https://reviews.llvm.org/D14030.
+    //
+    // Though it may be worth to try to revert those changes upstream, since the
+    // overhead of the initialization should be minor.
+    if sess.opts.cg.pgo_gen.is_some() && sess.target.target.options.linker_is_gnu {
+        cmd.args(&["-u".to_owned(), "__llvm_profile_runtime".to_owned()]);
+    }
+
     // FIXME (#2397): At some point we want to rpath our guesses as to
     // where extern libraries might live, based on the
     // addl_lib_search_paths