* src/vm/jit/codegen-common.cpp, src/vm/jit/x86_64/codegen.c: Generate

* src/vm/jit/codegen-common.cpp, src/vm/jit/x86_64/codegen.c: Generate
MFENCE instructions after (potential) volatile stores, augment patcher info
for patching them out again.
* src/vm/jit/codegen-common.hpp (codegen_emit_patchable_barrier): New function.
* src/vm/jit/patcher-common.cpp (patcher_add_patch_ref): Added return value.
* src/vm/jit/patcher-common.hpp: Added disp_mb field to patchref_t.
(patcher_add_patch_ref): Added return value.
* src/vm/jit/x86_64/codegen.h: Comment.
* src/vm/jit/x86_64/patcher.c: Patch out MFENCE for non-volatile writes.