hps: avoid yosys abc9 by default
hps: avoid yosys abc9 by default

It has started triggering an assertion failure in nextpnr, as
a workaround we can use the old abc instead.

Signed-off-by: Dan Callaghan <c6268a61d01f53ee8e097a439feb27b30b61efda@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground