To prepare for prototyping shared memory features, all internalized and
in-place internalizable (1- and 2-byte seq strings and external strings)
will always be allocated in the shared old space.

Cons strings, thin strings, and sliced strings remain allocated in the
thread-local space. They are copied over to the shared space when
internalized, as internalization implies flattening, which for these
strings requires a copy already.

To make the in-place internalization threadsafe, updating the map of
such strings is now done with a release store.

This CL does not yet support external strings.

Bug: v8:12007
Change-Id: I982c35c5120bf4c0c70c5294ce011b47430414c8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3140784
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77308}