This ensures that, when both sync and async FFs are available and abc9
is involved, the sync FFs will be used, and will thus remain available
for sequential synthesis.
I think these were probably missed by accident. Spotted because GCC
spits out lots of messages like this:
passes/techmap/dfflegalize.cc:114:7: warning: zero-length gnu_printf format string [-Wformat-zero-length]
114 | log("");
| ^~
(because we tell GCC that the first argument to log() looks like a
printf control string in log.h, and a zero length such string triggers
a warning).
