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

[Peter Wang]

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.
+
 NEWS for Mercury 20.01.2
 ========================
 
diff --git a/boehm_gc b/boehm_gc
index 5287fbab8..43ac2ea45 160000
--- a/boehm_gc
+++ b/boehm_gc
@@ -1 +1 @@
-Subproject commit 5287fbab8a6f2284a12beeb9e3ba592301436b98
+Subproject commit 43ac2ea45261ba0a715534e9da41b2504904c46a
-- 
2.25.0