[m-rev.] diff: Update to Boehm GC v8.2.4 and libatomic_ops v7.8.0

[Peter Wang]

On Wed, 12 Jul 2023 13:49:02 +1000 Peter Wang <novalazy at gmail.com> wrote:
> Remember to run "git submodule update --recursive" to bring
> the submodules in your workspace up to date.
> 
> --- 
> 
> .gitmodules:
> boehm_gc:
>     Update boehm_gc submodule to release-8_2-mercury-20230712.
> 

I've also deleted:
- the -rc tags from our bdwgc repository
- the release-7_6-mercury-20221006 tag from our libatomic_ops repository
  (it never got used)

Peter