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

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.
 
 # Base compiler configuration.
 config("compiler") {
   include_dirs = [ "//", root_gen_dir ]
   if (is_win) {
     cflags = [
       "/Gy",  # Enable function-level linking.
@@ skipping 21 matching lines @@
     # Stack protection.
     # TODO(brettw) why do we have different values for all of these cases?
     if (is_mac) {
       cflags += "-fstack-protector-all"
     } else if (is_chromeos) {
       cflags += "-fstack-protector-strong"
     } else if (is_linux) {
       cflags += [ "-fstack-protector", "--param=ssp-buffer-size=4" ]
     }
 
-    # Mac-specific compiler flags setup.
-    # ----------------------------------
     if (is_mac) {
+      # Mac-specific compiler flags setup.
+      # ----------------------------------
+
       # These flags are shared between the C compiler and linker.
       common_mac_flags = [
-        # TODO(brettw) obviously this arch flag needs to be parameterized.
-        "-arch i386",
-
         # Set which SDK to use.
         # TODO(brettw) this needs to be configurable somehow.
         "-isysroot", "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk",
 
         "-mmacosx-version-min=10.6",
       ]
 
+      # CPU architecture.
+      if (cpu_arch == "x64") {
+        common_mac_flags += "-arch x86_64"
+      } else if (cpu_arch == "x32") {
+        common_mac_flags += "-arch i386"
+      }
+
       cflags += common_mac_flags + [
         # Without this, the constructors and destructors of a C++ object inside
         # an Objective C struct won't be called, which is very bad.
         "-fobjc-call-cxx-cdtors",
       ]
 
       cflags_c += [ "-std=c99" ]
       cflags_cc += [ "-std=gnu++11" ]
 
       ldflags += common_mac_flags + [
         "-L.",
 
         # TODO(brettW) I don't understand these options.
         "-Wl,-rpath,@loader_path/.",
         "-Wl,-rpath,@loader_path/../../..",
       ]
+    } else {
+      # Non-Mac Posix compiler flags setup.
+      # -----------------------------------
+
+      # CPU architecture. We may or may not be doing a cross compile now, so for
+      # simplicity we always explicitly set the architecture.
+      if (cpu_arch == "x64") {
+        cflags += "-m64"
+        ldflags += "-m64"
+      } else if (cpu_arch == "x32") {
+        cflags += "-m32"
+        ldflags += "-m32"
+      }
     }
 
     # Linux-specific compiler flags setup.
     # ------------------------------------
     if (is_linux) {
       cflags += [
         "-fPIC",
         "-pthread",
         "-pipe",  # Use pipes for communicating between sub-processes. Faster.
       ]
@@ skipping 304 matching lines @@
   } else {
     cflags = [ "-g1" ]
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }