The issue is about linking x86_64 macOS executables statically against libavcodec, libavdevice, etc, which have some object files built against freestanding as they use YASM which isn't able to embed the macOS "tag" in the binary (see Building for macOS, but linking in object file built for free standing for detailed information on that issue).
Everything works fine under Xcode 11.6 - the following messages are here, but they are warnings. With the Xcode 12 update they become an error. How to make them warnings again ? I could not find anything relevant in the ld man page.
ld: in ffmpeg/lib/libavcodec.a(aacencdsp.o), building for macOS, but linking in object file built for free standing, for architecture x86_64