Was the last builds. Mac supports either, but only one at a time with the specific SDK needed for the architecture of the rack used for testing.
Linux is x86_64 only but with some fiddling about building rack free from source Linux can also be arm64. No official library though as only for official versions. Some modules are architecture specific but the demo module is not.
EDIT: I am not a Rosetta specialist. But you sound like you might need to zip up your x64 rack into an archive, and install rack free arm64 for testing?
EDIT2: I think to run a double SDK system has maybe issues with which is placed in the environment variables used, and merging both binaries into one usable by either. It sounds Apple.
EDIT3: x64 is likely the either but only choice as the older does not have the newer were as the newer may or may not have the older. So arm64 is beta test for natives, so you might lose module compatibility in some specific cases.
EDIT4: a goog slug could be “System 7” oh how a module named 68k would be funny. 