The heap sandbox mode was broken after the introduction of
WasmExportedFunctionData objects due to missing external pointer
handling. This CL implements that.

Bug: v8:10391
Change-Id: Icc6a2944b68f475c40b6431ab26400c35083b7bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2862771Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Samuel Groß <saelo@google.com>
Cr-Commit-Position: refs/heads/master@{#74415}