I have considered it. It would make sense, but I didn't feel like doing it 😄.
The current arch definition matches the use by webp, so a new key would need to be added to ARCHITECTURES.
...is what I was going to write, but looking at it now, webp in fact uses a different value for arm.
It might also make sense to build webp with CMake (which doesn't need the arch at all) instead of NMake; I'll take a look.

Edit: #7693