HotSpot Style Guide should permit alignas

Add alignas to the permitted features set, but only for variables, not for classes. Classes have the problem that dynamic allocation is unaware of alignment restrictions. (That's fixed in C++17. Though we might be able to add it ourselves for our allocators.)

Update ATTRIBUTE_ALIGNMENT to use alignas. We might deprecate the use of the macro, but changing its implementation quickly and cleanly applies the feature where the macro is being used.