It's Crossover that needs to be upgraded to a 64-bit on Macs to get rid of the pop-up warning, not WINE, which has been available in 64-bit builds for well over a year. Nor is it the Windows version of RootsMagic 7 (or earlier versions) as that's not relevant. Mac's know nothing about and nor do they care whether Windows Apps are 32-bit or 64-bit as they don't run those apps natively.
It's easy to build a 64-bit Wine.app using WINE directly, or with the help of one of the other 'front-end' apps available on MacOS/Linux, e.g. Wineskin (Mac Only), PlayOnMac or PlayOnLinux. Unfortunately, Crossover hasn't yet been upgraded to a 64-bit build, so will continue to produce the warnings until that happens; and unlike the other options available doesn't allow any choice of alternative WINE Engines for different apps, or to take advantage of bug-fixes in the ongoing development builds of WINE. You only get what's included in each Crossover release, the latest in Crossover 18.5 being Wine Version 4.0
Note: It is important to understand that it will still remain possible to run 32-bit Windows apps using WINE on future builds of MacOS providing the underpinning front end wrapper that is produced is recognised as a 64-bit app. In contrast it won't be possible to run a native 32-bit MacOS app which is what the pop-up warning is about! It's already possible in even more complex scenarios to retain 32-bit compatibility with no warning messages, i.e. running a full virtualised version of a 64-bit/32-bit Windows OS, older/same version of MacOS, 32-bit Linux etc., using one of the Virtualisation options (VirtualBox, Parallels, VMWare 64-bit) which are all 64-bit apps in their current versions and produce no pop-up compatibility warnings no matter what 'bittedness' the actual application is.