Rework how MB and GN handle concurrent links.

build/toolchain/mac/BUILD.gn

Index: build/toolchain/mac/BUILD.gn
diff --git a/build/toolchain/mac/BUILD.gn b/build/toolchain/mac/BUILD.gn
index 95f2bc3b179a5f8fa3d61457e9b582a0fd14ed3c..1fdc1eef3175805485521607d667a19981de86e3 100644
--- a/build/toolchain/mac/BUILD.gn
+++ b/build/toolchain/mac/BUILD.gn
@@ -15,6 +15,7 @@ assert(host_os == "mac")
 
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
+import("//build/toolchain/concurrent_links.gni")
 
 if (use_goma) {
   goma_prefix = "$goma_dir/gomacc "
@@ -40,6 +41,8 @@ template("mac_toolchain") {
     assert(defined(invoker.toolchain_os),
            "mac_toolchain() must specify a \"toolchain_os\"")
 
+    # concurrent_links is picked up from the declare_arg().
+
     # We can't do string interpolation ($ in strings) on things with dots in
     # them. To allow us to use $cc below, for example, we create copies of
     # these values in our scope.