Initial support for global safepoints kept it simple by entering a
safepoint for each of them one after another. This means
time-to-global-safepoint is the sum of all time-to-safepoint operations.
We can improve this slightly by splitting up the safepoint iteration
into two operations:

1) Initiate safepoint lock (locks local_heaps_mutex_, arms the barrier
   and sets SafepointRequested flag for all client threads)
2) Block until all runnning client threads reach a safepoint

We now perform operation 1) for all clients first and only then start
with operation 2).

Bug: v8:11708
Change-Id: Iaafd3c6d70bcf7026f722633e9250b04148b3da6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3310910Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78308}