[foreign+linkToNative] RFR: Add support for in memory returns to linkToNative invoker

[Jorn Vernee]

Hi,

I'd like to contribute a patch that adds support for in memory returns 
to the linkToNative invoker.

Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/inmemory/webrev.00/

All tests pass on Windows. The 2 ABIs are pretty much the same w.r.t. in 
memory returns, so this should work for SysV, but please give the tests 
a run as well.

Thanks,
Jorn