src: fix race on modpending

Fixes a rare race condition on modpending when two native modules are
loaded simultaneously on different threads by storing it thread-
locally.

PR-URL: https://github.com/nodejs/node/pull/21611


Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>