Is it possible to compile UNIX command line program for Apple Silicon architecture on Intel-based Mac?
The whole point of Universal Binaries is that you can compile the same code for 2 architectures, one of which (by necessity) is not the hardware you're running.
It has previously been possible to compile Universal Binaries for PPC and Intel, or 32-bit and 64-bit, regardless of the hardware doing the compiling.
Apple's documentation on porting to Apple Silicon (AS) is here:
https://developer.apple.com/documentation/xcode/porting_your_macos_apps_to_apple_silicon
Note the diagram which has the words &quot;Any Mac computer&quot;.

The Apple Silicon Mac mini is for developers to test that their AS code (and Apple's) runs well. There are no AS Macs available for purchase by non-developers (as of 14th October).
Apple has said that Intel binaries will run on Apple Silicon using Rosetta 2 emulation. So you will have at least the life-span of Big Sur before you need to create a Universal Binary.
I'd say you don't need to worry about this until:
a) AS Macs have been released,
b) people who have bought AS Macs are interested in your tool;
c) Xcode 12 is officially released (along with Big Sur)
at which time, compiling a Universal Binary for a command line tool should be pretty straightforward, just by changing some options.