AArch64: what now?

[Andrew Haley]

I've got to a point where I think the AArch64 Graal port is basically
decent.

However, there are some core Graal bugs which are blocking testing and
other progress.

These are:

 Constructors with final fields need a release memory barrier. #221

 Unsafe.storeFence is moved by Partial Escape Analysis #222

 WeakReference::get() read is elminated from a loop #223

I'd really appreciate help here.  221 has a baffling test failure, and
I've asked for clarification.  222, Partial Escape Analysis is more
than I really want to tackle right now.  223, I have asked for
clarification about whether a MembarNode(0) is an appropriate fix.

Thanks,

Andrew.