I am not sure if this is necessarily the correct approach, because I'm not sure if we can still use a single app target for "ios" and "iosmac".

We need to ship 3 .a files now instead of 2 because of the conflict for the x86_64 slice being used by both the iOS simulator and "UIKit for macOS".

# built for macOS (x86_64, i386)
s.macos.vendored_libraries = 'libs/mac/*.a'
# built for iOS and iOS simulator (armv7, arm64, x86_64, i386)
s.ios.vendored_libraries = 'libs/ios/*.a'
# built for "UIKit for macOS" (x86_64)
s.iosmac.vendored_libraries = 'libs/iosmac/*.a'

Originally posted by @chrisballinger in #8877 (comment)