I'm able to build one with Xcode 12.2 on an intel iMac pro with macOS Catalina 10.15.7
I added this on the gn command, then run ninja as shown on :
gn gen out/mac_release_arm64 --args='is_debug=false target_cpu="arm64"' --ide=xcode
ninja -C out/mac_release_arm64 mac_framework_objc
lipo -info out/mac_release_arm64/WebRTC.framework/Versions/A/WebRTC
Non-fat file: out/mac_release_arm64/WebRTC.framework/Versions/A/WebRTC is architecture: arm64
next I just manually combine this with the x86 version of the framework with lipo -create to make a universal fat binary