AArch64 port

[Andrew Haley]

FYI:

This is revised port.  A couple of bugs are fixed, and there is one
performance improvement which I thought was important.  It is the WB
stub, which no longer calls out to the C++ runtime code, but instead
evacuates the object as part of the stub.  Calling out to the runtime
in the write barrier is very expensive because we have a huge amount
of CPU state to save and restore.

http://cr.openjdk.java.net/~aph/aarch64-shenandoah-2/

Andrew.