Your approach is correct, unfortunately our Makefiles are not yet smart enough <https://github.com/JuliaLang/julia/issues/8312> to set OPENBLAS_DYNAMIC_ARCH=0 if you have overridden OPENBLAS_TARGET_ARCH, so you need to set that as well in order for your OPENBLAS_TARGET_ARCH flag to have any meaning.
Anything you learn about this topic that is not contained within DISTRIBUTING.md should be in there, so for example this little nugget of advice should probably go in there along with anything else you've struggled to figure out. Modifications to that file are welcome, just send a pull request and ping @staticfloat so I'll check it out. That file should be the repository of information regarding these topics, but it's likely out of date since it's been a while since it was written. -E
