GN: Declare ARM build arguments for either target_cpu or current_cpu == "arm"

build/config/arm.gni

Index: build/config/arm.gni
diff --git a/build/config/arm.gni b/build/config/arm.gni
index 996694aab32bd328820ce780459dcebf277213ec..c83cf48518b0c4db8bcf0f727568fabd34972e6a 100644
--- a/build/config/arm.gni
+++ b/build/config/arm.gni
@@ -2,7 +2,12 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-if (target_cpu == "arm") {
+# These are primarily relevant in current_cpu == "arm" contexts, where
+# ARM code is being compiled.  But they can also be relevant in the
+# host toolchain context when target_cpu == "arm", where a host-side
+# build tool being built will change its behavior depending on the
+# details of the target configuration.
+if (target_cpu == "arm" || current_cpu == "arm") {
   declare_args() {
     # Version of the ARM processor when compiling on ARM. Ignored on non-ARM
     # platforms.