How to switch implementations with compiler flags.

[takekikuchi]

I am currently trying to add functions and data types to Neon support for 
nimsimd(<https://github.com/guzba/nimsimd>). I want to distinguish in the 
source "when" block whether the compile target is arm32 or arm64 (--cpu in the 
compile flag). It would be better if it could also switch by ARM CPU core type 
(e.g. ARM generation). How can I do this?