GN: add target_arch build arg for android

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 7994dd77f78ac7470c173718d9219cf9417e0ece..890d33e0659e0d0b9ed1bf41b5e572979742b177 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -71,6 +71,16 @@ declare_args() {
     # 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"
 }
 
 # =============================================================================
@@ -174,6 +184,14 @@ if (is_win) {
   }
 }
 
+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
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