Rework v8_target_arch / target_cpu approach to use v8_target_cpu.

build/config/arm.gni

Index: build/config/arm.gni
diff --git a/build/config/arm.gni b/build/config/arm.gni
index d16c0d2a3b798168a2931286583aa333e3dea8b4..326a49ece16530816b979cacec9e70b265284c1b 100644
--- a/build/config/arm.gni
+++ b/build/config/arm.gni
@@ -2,12 +2,13 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/v8_target_cpu.gni")
+
 # 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") {
+# other contexts when the code will change its behavior based on the
+# cpu it wants to generate code for.
+if (current_cpu == "arm" || v8_target_cpu == "arm") {
   declare_args() {
     # Version of the ARM processor when compiling on ARM. Ignored on non-ARM
     # platforms.
@@ -86,7 +87,7 @@ if (target_cpu == "arm" || current_cpu == "arm") {
       arm_fpu = "vfpv3-d16"
     }
   }
-} else if (current_cpu == "arm64" || target_cpu == "arm64") {
+} else if (current_cpu == "arm64" || v8_target_cpu == "arm64") {
   # arm64 supports only "hard".
   arm_float_abi = "hard"
   arm_use_neon = true