GN: add target_arch build arg for android

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 53 matching lines @@
 
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
 
   if (os == "chromeos") {
     # Allows the target toolchain to be injected as arguments. This is needed
     # to support the CrOS build system which supports per-build-configuration
     # toolchains.
     cros_use_custom_toolchain = false
   }
+
+  # TODO(cjhopman): Make target_arch work for all platforms.
+
+  # Architecture of the target device. For Android builds, this will be equal to

[cjhopman, 2014/10/16 21:29:59]

This can't be guarded by (os == "android") because otherwise in a host toolchain
it isn't accessible (of course). The comment here reflects that it only affects
Android. If we are going to hook this up for other platforms, I guess we don't
really have to worry about it.

If we end up having a variable that we want to have accessible for target and
host toolchains for some configuration but not accessible at all for other
configurations, we would probably have to add something like target_os (just
like this target_arch).

+  # the cpu_arch of the default toolchain. When checking the CPU architecture
+  # for source files and build dependencies you should almost alway use cpu_arch
+  # instead. cpu_arch is the architecture of the current toolchain and allows
+  # cross-compiles (compiling the same target for multiple toolchains in the
+  # same build) to work.
+  target_arch = "arm"
 }
 
 # =============================================================================
 # OS DEFINITIONS
 # =============================================================================
 #
 # We set these various is_FOO booleans for convenience in writing OS-based
 # conditions.
 #
 # - is_android, is_chromeos, is_ios, and is_win should be obvious.
@@ skipping 83 matching lines @@
 if (is_win) {
   # Always use 32-bit on Windows, even when compiling on a 64-bit host OS,
   # unless the override flag is specified.
   if (force_win64) {
     cpu_arch = "x64"
   } else {
     cpu_arch = "x86"
   }
 }
 
+if (is_android) {
+  # TODO(cjhopman): enable this assert once bots are updated to not set
+  # cpu_arch.
+  #assert(cpu_arch == build_cpu_arch, "Android device target architecture should
+  #    be set with 'target_arch', not 'cpu_arch'")
+  cpu_arch = target_arch
+}
+
 # =============================================================================
 # SOURCES FILTERS
 # =============================================================================
 #
 # These patterns filter out platform-specific files when assigning to the
 # sources variable. The magic variable |sources_assignment_filter| is applied
 # to each assignment or appending to the sources variable and matches are
 # automatcally removed.
 #
 # Note that the patterns are NOT regular expressions. Only "*" and "\b" (path
@@ skipping 496 matching lines @@
       if (defined(invoker.output_extension)) { output_extension = invoker.output_extension }
       if (defined(invoker.output_name)) { output_name = invoker.output_name }
       if (defined(invoker.public)) { public = invoker.public }
       if (defined(invoker.public_configs)) { public_configs = invoker.public_configs }
       if (defined(invoker.public_deps)) { public_deps = invoker.public_deps }
       if (defined(invoker.sources)) { sources = invoker.sources }
       if (defined(invoker.visibility)) { visibility = invoker.visibility }
     }
   }
 }