Use llvm-ar when building with Clang.

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 d7e917af441fb6c7921c349c236404ca571dcfed..11cad83d0755bdf1bd448c63355f422942fef144 100644
--- a/build/config/posix/BUILD.gn
+++ b/build/config/posix/BUILD.gn
@@ -13,19 +13,6 @@ 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("$clang_base_path/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