External vectors of size 1 break the build on ice40 and Gowin

[zapta]

Discussion at upstream project here YosysHQ/nextpnr#1627

[zapta]

This issue exists for ICE40 and Gowin and is fixed for ECP5. Here is an Apio project for the three architectures that reproduces it.

work-verilog-vector.zip

[zapta]

Added an upstream issue YosysHQ/nextpnr#1628