Building Clang with a default target of arm64-apple-ios makes it default to arm64 macOS

If LLVM and Clang are build with `-DLLVM_DEFAULT_TARGET_TRIPLE=arm64-apple-ios7`, `clang -v` will report the target as arm64-apple-ios7 as expected, but compiling objects with -v will show the version getting passed to clang -cc1 is arm64-apple-macosx10.4.0, running `vtool -show` on the produced object file shows a platform of MACOS and a minos of 11.0. Passing `-target arm64-apple-ios7` or `-mios-version-min=7` makes clang use the iOS target as expected. This does not happen non-arm64 iOS targets, building with `-DLLVM_DEFAULT_TARGET_TRIPLE=armv7-apple-ios7` uses the iOS target by default as expected.