add Avian_java_util_concurrent_atomic_AtomicLong_VMSupportsCS8

Android's class library uses this to find out whether the VM supports
compareAndSwapLong natively.  Avian does on all platforms, so we just
return true.
Merge pull request #177 from dicej/VMSupportsCS8

add Avian_java_util_concurrent_atomic_AtomicLong_VMSupportsCS8