Rework how MB and GN handle concurrent links.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 06546f3da6624c0e998d657b09760dfb605085be..460ea223b92e14c1d44c870221778cf8575acc10 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -7,16 +7,7 @@ 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")
-}
+import("//build/toolchain/concurrent_links.gni")
 
 # This template defines a toolchain for something that works like gcc
 # (including clang).
@@ -118,8 +109,6 @@ template("gcc_toolchain") {
     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
     }