GN: Only honour system_libdir for target toolchain

Without this change when system_libdir is set (e.g.
to lib64) it would apply this to all toolchains
including the host toolchain.

Review URL: https://codereview.chromium.org/1583093002

Cr-Commit-Position: refs/heads/master@{#371875}

build/config/linux/pkg_config.gni

@@ -55,20 +55,24 @@ if (sysroot != "") {
     sysroot,
     "-a",
     current_cpu,
-    "--system_libdir",
-    system_libdir,
   ]
 } else if (pkg_config != "") {
   pkg_config_args = [
     "-p",
     pkg_config,
-    "--system_libdir",
-    system_libdir,
   ]
 } else {
   pkg_config_args = []
 }
 
+# Only use the custom libdir when building with the default toolchain.
+if (current_toolchain == default_toolchain) {
+  pkg_config_args += [
+    "--system_libdir",
+    system_libdir,
+  ]
+}
+
 template("pkg_config") {
   assert(defined(invoker.packages),
          "Variable |packages| must be defined to be a list in pkg_config.")