I’m sure Rosetta 2 is designed to work with gcc compiled apps but given that LLVM/Clang is largely sponsored by Apple, I’m sure the preference is on that, versus whatever version of gcc with whatever specific settings emacs is using.
I'm pretty sure the problem is that the "portable dumper" that replaces the dumpster fire that was unexec is not in fact portable across architectures.
Just a guess though.