Implement arflags in the GN build.

build/config/posix/BUILD.gn

Index: build/config/posix/BUILD.gn
diff --git a/build/config/posix/BUILD.gn b/build/config/posix/BUILD.gn
index fc5dd432103365e1b796e45f94c4a2603476c55f..c4198200862c9e805a98e09966d8f2c73230ef76 100644
--- a/build/config/posix/BUILD.gn
+++ b/build/config/posix/BUILD.gn
@@ -2,7 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/sysroot.gni")
+import("//build/toolchain/toolchain.gni")
 
 assert(is_posix)
 
@@ -10,6 +12,20 @@ group("posix") {
   visibility = [ "//:optimize_gn_gen" ]
 }
 
+# This is included by reference in the //build/config/compiler config that
+# is applied to all Posix targets. It is here to separate out the logic that is
+# Posix-only. Note that this is in addition to an OS-specific variant of this
+# config.
+config("compiler") {
+  if ((allow_posix_link_time_opt || is_cfi) && !is_nacl) {
+    arflags = [
+      "--plugin",
+      rebase_path("//third_party/llvm-build/Release+Asserts/lib/LLVMgold.so",
+                  root_build_dir),
+    ]
+  }
+}
+
 # This is included by reference in the //build/config/compiler:runtime_library
 # config that is applied to all targets. It is here to separate out the logic
 # that is Posix-only. Please see that target for advice on what should go in