Lookups and updates to the executable_memory_ unordered_map need to be
protected with mutex.

Bug: v8:10315, v8:10546
Change-Id: Ic17e19d1e4fda18b99103a96052940e68e970586
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208867Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67902}