gn: define and use clang_base_path

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 ccb2364c5fdf128c3f7bd5f48d9482b10e96c18c..d7e917af441fb6c7921c349c236404ca571dcfed 100644
--- a/build/config/posix/BUILD.gn
+++ b/build/config/posix/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/sysroot.gni")
 import("//build/toolchain/toolchain.gni")
@@ -20,8 +21,7 @@ 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),
+      rebase_path("$clang_base_path/lib/LLVMgold.so", root_build_dir),
     ]
   }
 }