on MacOS part of the ZUNO toolchain is not 64bit compatible so you have to manually update SDCC. Download it from sourceforge.
locate your Arduino Library and in particular the zuno toolchain. It should be there:
/Users/your_username/Library/Arduino15/packages/Z-Uno/tools/zuno_toolchain/00.08.80/zuno_toolchain/sdcc
replace binary files in this directory with the ones from sourceforge.