_yosys: handle unparseable versions

_yosys: handle unparseable versions

Do not use yosys binaries with unparseable version numbers. This ensures
that nmigen always knows what version of yosys it is generating RTLIL
for.

The effect of this change is that if the version number of the system
yosys is unparsable, nmigen will attempt to fallback to the builtin
Yosys.

Fixes #409.