Avoid the C-call to switch the flag, just store to the address
directly. Since js-to-wasm wrappers are still isolate dependent,
we just store the address of the thread-local flag in the isolate
(in ThreadLocalTop) and update it if threads are switched.

R=ahaas@chromium.org, mstarzinger@chromium.org

Bug: chromium:862123, v8:5277
Change-Id: I9e8a40094f11a8b3ba6701dfa7fa026a2d052cb1
Reviewed-on: https://chromium-review.googlesource.com/1136299
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54531}