GN: Only honour system_libdir for target toolchain

build/config/linux/pkg_config.gni

Index: build/config/linux/pkg_config.gni
diff --git a/build/config/linux/pkg_config.gni b/build/config/linux/pkg_config.gni
index 58769e26958ac066b0ed729a8dae6452739b9728..204cbab1d3be382984e1ac2d32ee40e7ddba3043 100644
--- a/build/config/linux/pkg_config.gni
+++ b/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.")