[macOS] Enable universal binary builds by default (#100271)

This changes the default build architectures for Flutter macOS apps to
x86_64 and arm64. Previously, we manually excluded arm64 builds via the
EXCLUDE_ARCHS Xcode setting in Flutter's generated xcconfig file. This
eliminates setting EXCLUDE_ARCHS during the build and updates the
default architectures in the tool and in the macos_assemble.sh wrapper.

Issue: https://github.com/flutter/flutter/issues/97681
Umbrella issue: https://github.com/flutter/flutter/issues/60113