[ext-code-space] Add CodeDataContainer::code field and friends

... behind the v8_enable_external_code_space build flag.

This is a first CL in a row of CLs that will make CodeDataContainer
the only type of objects that could contain references to Code objects
(besides the Code objects embedded into the generated code).
Eventually these changes will allow us to move Code space out of the V8
heap cage.

This CL adds |code| field to ensure that CodeDataContainer keeps the
respective Code object alive and |code_entry_point| field that contains
cached value of the code().InstructionStart().

Bug: v8:11880
Change-Id: Ie7ce75667d8da306797d203691b429671bc4530d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2964093


Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75179}