[m-rev.] diff: Update to Boehm GC v8.2.12 and libatomic_ops v7.8.4

[Peter Wang]

In particular, this fixes a 'REFERENCED_DYNAMICALLY flag' linker warning
on macOS. <https://github.com/bdwgc/bdwgc/issues/733>

.gitmodules:
boehm_gc:
    Update boehm_gc submodule to release-8_2-mercury-20261006.

NEWS.md:
    Announce change.

diff --git a/NEWS.md b/NEWS.md
index 5f77818732..e6668cdd10 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1991,7 +1991,7 @@ Changes to the Mercury deep profiler
 Changes to the Mercury implementation
 -------------------------------------
 
-* We have upgraded the bundled Boehm GC to v8.2.8 and libatomic_ops to v7.8.2.
+* We have upgraded the bundled Boehm GC to v8.2.12 and libatomic_ops to v7.8.4.
 
 Portability improvements
 ------------------------
diff --git a/boehm_gc b/boehm_gc
index abb522c8f2..1da8577253 160000
--- a/boehm_gc
+++ b/boehm_gc
@@ -1 +1 @@
-Subproject commit abb522c8f29daf56820e1e02c3b93fe8f669412f
+Subproject commit 1da857725358791aeafe698a0be901c1fb0fce4a
-- 
2.54.0