[iOS/GN] Allow compilation with system clang.

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 195 matching lines @@
 } else if (target_os == "chromeos") {
   # See comments in build/toolchain/cros/BUILD.gn about board compiles.
   assert(host_os == "linux",
          "ChromeOS builds are only supported on Linux hosts.")
   if (is_clang) {
     _default_toolchain = "//build/toolchain/cros:clang_target"
   } else {
     _default_toolchain = "//build/toolchain/cros:target"
   }
 } else if (target_os == "ios") {
-  _default_toolchain = "//build/toolchain/mac:ios_clang_arm"
+  _default_toolchain = "//build/toolchain/mac:ios_clang_$target_cpu"
 } else if (target_os == "linux") {
   if (is_clang) {
     _default_toolchain = "//build/toolchain/linux:clang_$target_cpu"
   } else {
     _default_toolchain = "//build/toolchain/linux:$target_cpu"
   }
 } else if (target_os == "mac") {
   assert(host_os == "mac", "Mac cross-compiles are unsupported.")
   _default_toolchain = host_toolchain
 } else if (target_os == "win") {
@@ skipping 431 matching lines @@
     forward_variables_from(invoker, "*", [ "visibility" ])
 
     # All shared libraries must have the sanitizer deps to properly link in
     # asan mode (this target will be empty in other cases).
     if (!defined(deps)) {
       deps = []
     }
     deps += [ "//build/config/sanitizers:deps" ]
   }
 }