[m-rev.] for review: Upgrade Boehm GC to v7.6.2

[Peter Wang]

I've only tested this some on Linux (glibc, musl).

----

boehm_gc:
    Update boehm_gc submodule to release-7_6-mercury-20180108

diff --git a/.gitmodules b/.gitmodules
index 4e3c94bfa..071fda5ef 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,5 +1,5 @@
 [submodule "boehm_gc"]
 	path = boehm_gc
 	url = ../bdwgc.git
-	branch = release-7_6-mercury
+	branch = release-7_6_2-mercury
 	update = checkout
diff --git a/boehm_gc b/boehm_gc
index 81ac1757f..038854e48 160000
--- a/boehm_gc
+++ b/boehm_gc
@@ -1 +1 @@
-Subproject commit 81ac1757fad49c1fe9237d29eaa7e5462d6bd86f
+Subproject commit 038854e489dd1da22c81c550f1c031966ff8da92
-- 
2.15.1