Instead of defining one global (FLAG_foo) per flag, define all flag
values as fields in a global {v8_flags} struct. This guarantees that the
memory is contiguous, and together with proper alignment allows us to
later memory-protect that memory space.

In order to avoid rewriting all existing code that uses the {FLAG_foo}
syntax, we define global aliases: {FLAG_foo} is a reference to
{v8_flags.foo}.

After the next branch cut (v10.6), follow-up CLs will rewrite all
existing code to use the {v8_flags.foo} syntax, and after another branch
cut (v10.7) the aliases will be removed.
This should allow us to merge back most fixes to the previous branch
(N-1). Merges to stable (N-2) might still require resolving merge
conflicts manually, if they modify code that reads flags.

R=cbruni@chromium.org
CC=​sroettger@chromium.org

Bug: v8:12887
Change-Id: I8bc44429767f611484fe345d7268af1d55c98124
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3810187
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82423}