[gn] Make v8_target_arch a GN declare_arg()

build_overrides/v8.gni

Index: build_overrides/v8.gni
diff --git a/build_overrides/v8.gni b/build_overrides/v8.gni
index af5cbc851143b1c140876e8d5efef49ed2781ff9..719379d213877224afe125e5f4d5561d82726afd 100644
--- a/build_overrides/v8.gni
+++ b/build_overrides/v8.gni
@@ -4,16 +4,12 @@
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
+import("//gni/v8.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
-declare_args() {
-  # V8 generates code for this architecture.
-  v8_target_arch = target_cpu
-}
-
 if (((v8_target_arch == "ia32" ||
       v8_target_arch == "x64" ||
       v8_target_arch=="x87") &&