Rework v8_target_arch / target_cpu approach to use v8_target_cpu.

build/config/v8_target_cpu.gni

+# Copyright 2016 The Chromium Authors. 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() {
+  # This arg is used when we want to tell the JIT-generating v8 code
+  # that we want to have it generate for an architecture that is different
+  # than the architecture that v8 will actually run on; we then run the
+  # code under an emulator. For example, we might run v8 on x86, but
+  # generate arm code and run that under emulation.
+  #
+  # This arg is defined here rather than in the v8 project because we want
+  # some of the common architecture-specific args (like arm_float_abi or
+  # mips_arch_variant) to be set to their defaults either if the current_cpu
+  # applies *or* if the v8_target_cpu applies.
+  #
+  # TODO(crbug.com/620527) - rework this whole approach so that it isn't
+  # v8-specific.
+  v8_target_cpu = target_cpu

[Michael Achenbach, 2016/06/16 07:49:55]

This logic:
https://cs.chromium.org/chromium/src/v8/gni/v8.gni?l=24

should move here too. Maybe with an extra check that target_cpu==x64, otherwise
v8_target_cpu arm64 makes no sense.

[Dirk Pranke, 2016/06/16 17:35:05]

Done, in the interest of moving forward, but I was wondering about this ... does
this mean that the normal x64 JIT path doesn't work correctly under MSAN? That
seems bad ...

+}