The getters and setters were not using the correct types for the casts
of {valptr_} and {defptr_}. It was mostly fine though, because
{FlagValue<T>} just wraps a {T}, so accessing a {FlagValue<T>*} as a
{T*} just works.

This CL fixes the casts of {valptr_} to use proper {FlagValue<T>*}, and
changes the definition of the default values to use plain {T} instead of
{FlagValue<T>}.

R=tebbi@chromium.org

Bug: v8:12887
Change-Id: I3a38ba466df95a4c7b45e83fbd5d37c9a4785a13
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695558Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81029}