Updating truffle in graal-core

[Jaroslav Tulach]

Hi.
I'd like to update the truffle dependency in graal-core to 
https://github.com/graalvm/truffle/commit/
f7a3ac94326a030806fc6c091fe545dd0c8498cb
it contains many robustness improvements in Java/Truffle interop and I consider 
it important for all projects to get them.

-jt

PS: Once in I will be able to remove at least one hack in downstream projects.