[foreign-abi] RFR 8233317: Add the ProgrammableInvoker

[Jorn Vernee]

Hi,

This is an RFR for the patches posted to the "On Invokers" thread [1]. 
It is a merge of the 3 patches for the different ABIs, made by Maurizio, 
Nick, and myself. Attribution will be given through the "Contributed-by" 
clause of the commit message.

Bug: https://bugs.openjdk.java.net/browse/JDK-8233317
Webrev: http://cr.openjdk.java.net/~jvernee/prog_back_final/webrev.00/

There is more work that can still be done around this area, but since 
all the tests are passing I think this is a good first version. Some 
items still on the TODO list:
- Factor out IMR adapter code into shared utility.
- Make ABIDescriptor/BufferLayout code in the VM shared, if possible.
- Revisit register spill/fill code in upcall/downcall stub.

On the other hand, we discovered some holes in the tests which revealed 
some bugs that are now fixed, and some bugs were left behind in the old 
implementation [2], so I think overall the ProgrammableInvoker has 
undergone more testing than the old backend had.

Thanks,
Jorn

[1] : 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-September/006347.html
[2] : https://bugs.openjdk.java.net/browse/JDK-8231502