GN: Change the default meaning of dart_target_arch from current_cpu to target_cpu.

runtime/runtime_args.gni

 # Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 declare_args() {
   # Instead of using is_debug, we introduce a different flag for specifying a
   # Debug build of Dart so that clients can still use a Release build of Dart
   # while themselves doing a Debug build.
   dart_debug = false
 
   # Set the runtime mode. This affects how the runtime is built and what
   # features it has. Valid values are:
   # 'develop' (the default) - VM is built to run as a JIT with all development
   # features enabled.
   # 'profile' - The VM is built to run with AOT compiled code with only the
   # CPU profiling features enabled.
   # 'release' - The VM is built to run with AOT compiled code with no developer
   # features enabled.
   #
   # These settings are only used for Flutter, at the moment. A standalone build
   # of the Dart VM should leave this set to "develop", and should set
   # 'is_debug', 'is_release', or 'is_product'.
   #
   # TODO(rmacnak): dart_runtime_mode no longer selects whether libdart is build
   # for JIT or AOT, since libdart waw split into libdart_jit and
   # libdart_precompiled_runtime. We should remove this flag and just set
   # dart_debug/dart_product.
   dart_runtime_mode = "develop"
 
-  # Explicitly set the target architecture in case of precompilation. Leaving
-  # this unspecified results in automatic target architecture detection.
+  # Explicitly set the target architecture to use a simulator.
   # Available options are: arm, arm64, x64, ia32, and dbc.
-  dart_target_arch = ""
+  dart_target_arch = target_cpu
 
   # The optimization level to use for debug builds.
   dart_debug_optimization_level = "2"
 
   # Whether to fall back to built-in root certificates when they cannot be
   # verified at the operating system level.
   dart_use_fallback_root_certificates = false
 
   # The BUILD.gn file that we pull from chromium as part of zlib has a
   # dependence on //base, which we don't pull in. In a standalone build of the
   # VM, we set this to //runtime/bin/zlib where we have a BUILD.gn file without
   # a dependence on //base.
   dart_zlib_path = "//third_party/zlib"
 
   # Whether to link the standalone VM against tcmalloc. The standalone build of
   # the VM enables this only for Linux builds.
   dart_use_tcmalloc = false
 }