Specify SDK version explicitly when linking for iOS
Specify SDK version explicitly when linking for iOS

If `-sdk_version` option is absent, ld infers the version from
`-syslibroot` path.

Since sysroot directory name in our dependencies differs from the
Xcode SDK names, the resulting binary didn't contain correct SDK version
specification, and this was leading to device-orientation-related bugs
on iOS 11.

wiltonlazary/kotlin-native,JuliusKunze/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,wiltonlazary/kotlin-native,wiltonlazary/kotlin-native,JuliusKunze/kotlin-native,JuliusKunze/kotlin-native,JuliusKunze/kotlin-native,wiltonlazary/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,JuliusKunze/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,wiltonlazary/kotlin-native,JetBrains/kotlin-native