Attach to the shared isolate after the main thread was set up. Otherwise
it could happen that a shared GC initiated from another isolate might
see no threads are running and performs the safepoint operation in the
middle of isolate deserialization.

We use DisallowSafepoints to check that the isolate doesn't join a
global safepoint before deserialization is complete. DisallowSafepoints
used to prevent only invocations of Safepoint() but was updated to
also prevent Park() and Unpark() invocations. Each state change could
cause the thread to reach a safepoint, which would allow a shared GC
to run.

We now also DCHECK that every isolate has at least one local heap and
that shared collections aren't started before deserialization is
complete.

Bug: v8:11708
Change-Id: Iba3fb59dd951d5ee4fc9934158062287302fc279
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3221157
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77424}