Zero runtime can lock-up on linux-alpha

We have observed that the Zero runtime can lock up on alpha, especially on SMP machines running Linux. This manifests itself in the OpenJDK taking forever by repeatedly printing "compiler/java/jar running ..." and never finishing.

It can be worked around by booting the machine with a uni-processor kernel. The fix is to use __sync_synchronize() even for the LIGHT_MEM_BARRIER in src/os_cpu/linux_zero/vm/orderAccess_linux_zero.inline.hpp.