Explictly pass -marm when arm mode is required

build/config/compiler/BUILD.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.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 767 matching lines @@
   if (current_cpu == "arm" && arm_use_thumb && is_posix &&
       !(is_mac || is_ios || is_nacl)) {
     cflags = [ "-mthumb" ]
     if (is_android && !is_clang) {
       # Clang doesn't support this option.
       cflags += [ "-mthumb-interwork" ]
     }
   }
 }
 
+config("compiler_arm") {
+  if (current_cpu == "arm" && is_chromeos) {
+    # arm is normally the default mode for clang, but on chromeos a wrapper
+    # is used to pass -mthumb, and therefor change the default.
+    cflags = [ "-marm" ]
+  }
+}
+
 # runtime_library -------------------------------------------------------------
 #
 # Sets the runtime library and associated options.
 #
 # How do you determine what should go in here vs. "compiler" above? Consider if
 # a target might choose to use a different runtime library (ignore for a moment
 # if this is possible or reasonable on your system). If such a target would want
 # to change or remove your option, put it in the runtime_library config. If a
 # target wants the option regardless, put it in the compiler config.
 
@@ skipping 841 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }