This CL fixes a null dereference when an attempt is made to access
the current arm64 simulator from a background thread.

Bug: chromium:1267491
Change-Id: I9232fe134fccbff162eb5076aff20884872e4cc7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3264219
Auto-Submit: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77757}