This removes atomicops_internals_portable.h and inlines
atomicops_internals_std.h into atomicops.h.

Change-Id: Id06cae42a277fee9379590ca755571193f9e8bbc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2848462Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74152}