Pass the macOS deployment target to swiftc in Metal builds

[Slymaster]

I bundle a Metal build of KataGo in a small macOS app and noticed the binary needed macOS 26, the version of my build Mac, even though CMakeLists.txt sets CMAKE_OSX_DEPLOYMENT_TARGET to 13.0.

The C++ files do get -mmacosx-version-min=13.0, but CMake passes nothing to swiftc, and swiftc is what links katago, so it falls back to the host's version. Passing -DCMAKE_OSX_DEPLOYMENT_TARGET=13.0 on the command line doesn't help either.

This adds -target <arch>-apple-macosx13.0 to the Swift flags, unless a -target is already in CMAKE_Swift_FLAGS.

Tested on Apple Silicon only (M2 Max, macOS 26.6.2, Xcode 27.0, CMake 4.4.3), checking minos with otool -l katago:

• current master: 26.0
• with this change: 13.0
• with this change and -DCMAKE_Swift_FLAGS="-target arm64-apple-macosx14.0": 14.0

The 13.0 build runs fine (katago version reports the Metal backend, and katago benchmark works).