Fix FGPA synthesis on latest yosys toolchain

This may look like using nextpnr instead of arachne-pnr. 

Currently synthesis works using 
```
$ arachne-pnr -v
arachne-pnr 0.1+310+0 (git sha1 5d830dd, g++ 7.3.0-27ubuntu1~18.04 -O2)
$ yosys -V
Yosys 0.7+612 (git sha1 8b92ddb9, clang 6.0.0-1ubuntu2 -fPIC -Os)
```
but not when using later versions.