In the simulators, the ExternalReference constructor rewrites external
addresses, which involves mutating a linked list rooted in the isolate.

We already construct external references concurrently (at least in Turbofan),
but the list mutation was not thread-safe (though no crashes are known). This
CL adds the necessary locking.

BUG=v8:6048

Review-Url: https://codereview.chromium.org/2852983002
Cr-Commit-Position: refs/heads/master@{#45014}