[foreign] RFR 8212987 : Binder should allows negative values for native pointers

[Jorn Vernee]

Hello,

Please review this patch which tweaks the binder to allow negative 
values for native pointers.

The problem was previously discussed here: 
http://mail.openjdk.java.net/pipermail/panama-dev/2018-October/002994.html

Bug : https://bugs.openjdk.java.net/browse/JDK-8212987
Webrev : http://cr.openjdk.java.net/~jvernee/panama/webrevs/8212987/

As a reminder, I'm not a committer, so someone else will have to push 
this.

Thanks,
Jorn