Simply removing that flag isn't the way to go.
You need to add a conditional statement to that block, and may need to check other sections as well.
Or create a new .mak file tailored to arm64 (not the best way to go about it, but if you're gonna take the brute-force approach, that's one way to do it while preserving the original .mak file so it's still portable).
If you want this bad enough, you'll find a way....
AIR.