[m-rev.] for post-commit review: Upgrade Boehm GC to v7.6.12 and libatomic_ops to v7.6.8

[Julien Fischer]

Hi Peter,

On Thu, 2 Apr 2020, Peter Wang wrote:

> boehm_gc:
>    Update boehm_gc submodule to release-7_6-mercury-20190402.
>
> NEWS:
>    Announce change.
>
> diff --git a/NEWS b/NEWS
> index 00865c7bb..4d2ba06e1 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -244,6 +244,8 @@ Changes to the Mercury implementation
>   Another consequence is that the `--trail-size` and `--trail-size-kwords`
>   runtime options no longer have any effect, and are deprecated.
> 
> +* We have upgraded the bundled Boehm GC to v7.6.12 and libatomic_ops to v7.6.8.

If the upgrade fixes any specific issues with Mercury you should list
them here as well.

As I recall, there's some git submodule magic required to update
existing workspaces.  Could you please remind us (well, me at least!)
what that is?

Otherwise, that's fine.

Julien.