Update mmtk-core

Author: qinsoon

mmtk/Cargo.toml

@@ -26,7 +26,7 @@ lazy_static = "1.1"
 # - change branch
 # - change repo name
 # But other changes including adding/removing whitespaces in commented lines may break the CI.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "dde48d1a056046459aabad0ffbd28fd400a28bf7" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "612ee339ce909cca7b74857dc62e16da5eb5a3f3" }
 # Uncomment the following to build locally
 # mmtk = { path = "../repos/mmtk-core" }