You need to install the 64bit version of Nim to match your new gcc. If you didn't have gcc when you started, choosenim would install 32bit by default. This was recently improved but you probably have an older choosenim.
Regardless of arch, the problem is the dll so please try copying it to the same dir as the exe and try that. You don't need to fiddle with arch unless you have a specific need that depends on arch.
