or1200-generic: update verilator support

This patch removes the local copy of elf-loader in or1200-generic.
This copy is no longer needed because fusesoc now adds verilator
files from all cores.

The verilator testbench is also cleaned-up and adapted to the
up-to-date elf-loader.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>