GN: Add support for 32- and 64-bit cross-compiles.

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.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 365 matching lines @@
 # ==============================================================================
 # TOOLCHAIN SETUP
 # ==============================================================================
 #
 # Here we set the default toolchain, as well as the variable host_toolchain
 # which will identify the toolchain corresponding to the local system when
 # doing cross-compiles. When not cross-compiling, this will be the same as the
 # default toolchain.
 
 if (is_win) {
-  if (cpu_arch == "x64") {
+  if (build_cpu_arch == "x64") {
     host_toolchain = "//build/toolchain/win:64"
-  } else if (cpu_arch == "x86") {
+  } else if (build_cpu_arch == "x86") {
     host_toolchain = "//build/toolchain/win:32"
   }
-  set_default_toolchain(host_toolchain)
+
+  if (cpu_arch == "x64") {
+    set_default_toolchain("//build/toolchain/win:64")
+  } else if (cpu_arch == "x86") {
+    set_default_toolchain("//build/toolchain/win:32")
+  }
 } else if (is_linux) {
-  host_toolchain = "//build/toolchain/linux:host"
-  if (cpu_arch == "arm" && build_cpu_arch != "arm") {
-    # Special toolchain for ARM cross-compiling.
-    set_default_toolchain("//build/toolchain/linux:arm-cross-compile")
-  } else {
-    # Use whatever GCC is on the current platform.
-    set_default_toolchain(host_toolchain)
+  if (build_cpu_arch == "arm") {
+    host_toolchain = "//build/toolchain/linux:arm"
+  } else if (build_cpu_arch == "x86") {
+    host_toolchain = "//build/toolchain/linux:32"
+  } else if (build_cpu_arch == "x64") {
+    host_toolchain = "//build/toolchain/linux:64"
+  }
+
+  if (build_cpu_arch == "arm") {
+    set_default_toolchain("//build/toolchain/linux:arm")
+  } else if (build_cpu_arch == "x86") {
+    set_default_toolchain("//build/toolchain/linux:32")
+  } else if (build_cpu_arch == "x64") {
+    set_default_toolchain("//build/toolchain/linux:64")
   }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 }