V8 uses "thread_local" since several years now, so we can assume it to
be available on all platforms. This avoids a cumbersome macro
definition.

R=ahaas@chromium.org

Bug: v8:11755
Change-Id: I64826188a15a22238ffbcb3215a5b5d66470d573
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2885038Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74585}