--shared-string-table assumes that all old strings are in the shared
heap. However, when also using --expose-gc we create an external string
for the GC function name. So far external strings are always allocated
in the local old space though, which results in a heap verification
error. This CL creates external string in the shared old heap with
--shared-string-table enabled.

In order to pass all the tests this CL also has to:

* Stop marking into the shared heap for VisitEmbeddedPointer and
  VisitCodePointer.
* Relax DCHECK in String::GetFlatContent: We cannot check the thread
  id for any shared string. Even if that string isn't really shared atm.

Bug: v8:11708
Change-Id: I51fec5ba038d035be5fe5e1277ef9286efc8dc2a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3447371Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79022}