Rework how MB and GN handle concurrent links.

build/toolchain/gcc_toolchain.gni

 # Copyright (c) 2013 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.
 
 import("//build/config/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
-# "concurrent_links" is a toolchain variable. By computing it here rather than
-# inside the toolchain, the exec_script will only get run once rather than
-# each time the toolchain template is invoked.
-if (allow_posix_link_time_opt || is_cfi) {
-  concurrent_links_ =
-      exec_script("get_concurrent_links.py", [ "--lto" ], "value")
-} else {
-  concurrent_links_ = exec_script("get_concurrent_links.py", [], "value")
-}
-
 # This template defines a toolchain for something that works like gcc
 # (including clang).
 #
 # It requires the following variables specifying the executables to run:
 #  - ar
 #  - cc
 #  - cxx
 #  - ld
 # and the following which is used in the toolchain_args
 #  - toolchain_cpu  (What "current_cpu" should be set to when invoking a
@@ skipping 80 matching lines @@
   toolchain(target_name) {
     assert(defined(invoker.ar), "gcc_toolchain() must specify a \"ar\" value")
     assert(defined(invoker.cc), "gcc_toolchain() must specify a \"cc\" value")
     assert(defined(invoker.cxx), "gcc_toolchain() must specify a \"cxx\" value")
     assert(defined(invoker.ld), "gcc_toolchain() must specify a \"ld\" value")
     assert(defined(invoker.toolchain_cpu),
            "gcc_toolchain() must specify a \"toolchain_cpu\"")
     assert(defined(invoker.toolchain_os),
            "gcc_toolchain() must specify a \"toolchain_os\"")
 
-    concurrent_links = concurrent_links_
-
     if (defined(invoker.cc_wrapper)) {
       cc_wrapper = invoker.cc_wrapper
     }
     if (defined(invoker.use_goma)) {
       use_goma = invoker.use_goma
     }
     if (use_goma) {
       assert(cc_wrapper == "", "Goma and cc_wrapper can't be used together.")
       compiler_prefix = "$goma_dir/gomacc "
     } else if (cc_wrapper != "") {
@@ skipping 393 matching lines @@
                              "toolchain_os",
                              "use_gold",
                              "strip",
                            ])
 
     if (defined(invoker.use_debug_fission)) {
       use_debug_fission = invoker.use_debug_fission
     }
   }
 }