unexpected allocation of Binding.Context in downcall method handle

[Maurizio Cimadamore]

Hi,
following the discussion on [1], here is a quick patch which attempts to 
remove the additional allocation:

https://github.com/mcimadamore/jdk/pull/new/avoid_context_alloc

I'm not able to see any gain using synthetic benchmarks, but perhaps 
worth a try to confirm that this is really the issue we're seeing?

Thanks
Maurizio

[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2021-December/015938.html