…ackends.

…tered.

This way, exceptions can be properly propagated from upcalls to
downcalls even when mixing different NFI backends (e.g. downcalling with
libffi and upcalling with panama). It's also removing some duplicated
code.

…ptions for later re-throwing.

* These reads are unused for most types except closure types,
  so this only helps for closure types, otherwise Graal removes the
  unused reads in High Tier after lowering.