cores: Fix PCINT8 for ATmega128RFA1 and ATmega128RFR2

PCINT8 was previously not setup. Per the spec sheet it is on Port E pin
0.
Merge pull request #73 from cardoe/rfa1-rfr2-pcint8

cores: Fix PCINT8 for ATmega128RFA1 and ATmega128RFR2
make: Link libelf in libsimavr.so.1 target

The build (sometimes) fails if libsimavr.so.1 is not linked with
libelf (see https://bugs.archlinux.org/task/40309).
Update README.md

Lots of cores were missing in the readme