[build, native] Adjust buildscripts after dropping K/N targets (6/8)

- Restore PlatformManager constructor (used in buildscripts)
- Replace references to deprecated K/N target with 'else'

Why: buildscripts use these references and compile against older K/N
version (from bootstrap). So they can't be adjusted to newer API,
compilation will fail right away. However, they can't be kept without
changes, because bootstrap tests will fail.

^KT-64517
^KT-66500

native/utils/src/org/jetbrains/kotlin/konan/target/Platform.kt

@@ -65,4 +65,3 @@ class PlatformManager private constructor(private val serialized: Serialized) :
         private fun readResolve(): Any = PlatformManager(this)
     }
 }
-