It seems that at a certain point (no backtrace is provided to the user) it just tries to:
-- Build files have been written to: /Users/minuscorp/Documents/GitHub/build/LLVMClangSwift_iphoneos/swift-macosx-x86_64 + popd ~/Documents/GitHub + /usr/local/bin/cmake --build /Users/minuscorp/Documents/GitHub/build/LLVMClangSwift_iphoneos/swift-macosx-x86_64 -- -j12 all swift-test-stdlib-macosx-x86_64 ninja: error: unknown target 'swift-test-stdlib-macosx-x86_64' ./swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting ./swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting
And that's all. Maybe the preset is outdated and need an update. It is important to us to have toolchains that can be used on devices (due to certain limitations).