[AVR] Add FeatureEIJMPCALL to FamilyAVR6

This feature was probably missed when adding FamilyAVR6, but should
definitely be there. I checked all four devices in the AVR6 family and
they all support eijmp/eicall.

Found while working on https://reviews.llvm.org/D137572.

Differential Revision: https://reviews.llvm.org/D137573