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..b83ebce60917964095c7857c4fc61165504f6f47 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -71,6 +71,11 @@ declare_args() {
     # toolchains.
     cros_use_custom_toolchain = false
   }
+
+  if (os == "android") {
+    # Architecture of the target device.

[brettw, 2014/10/16 19:19:07]

Can you add here something to the effect of "This will be equal to the cpu_arch
of the default toolchain. When checking the CPU architecture for source files
and build dependencies, you should almost always use cpu_arch instead. cpu_arch
is the architecture of the current toolchain, and using it allows cross-compiles
(compiling the same target for multiple toolchains in the same build) to be
supported.

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

Done.

+    target_arch = "arm"
+  }
 }
 
 # =============================================================================
@@ -174,6 +179,14 @@ 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
 # =============================================================================