[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

kotlin-native/runtime/build.gradle.kts

@@ -96,9 +96,8 @@ bitcode {
         module("libbacktrace") {
             val elfSize = when (target.architecture) {
                 TargetArchitecture.X64, TargetArchitecture.ARM64 -> 64
-                TargetArchitecture.X86, TargetArchitecture.ARM32,
+                TargetArchitecture.X86, TargetArchitecture.ARM32 -> 32
-                TargetArchitecture.MIPS32, TargetArchitecture.MIPSEL32,
+                else -> 32 // TODO(KT-66500): remove after the bootstrap
-                TargetArchitecture.WASM32 -> 32
             }
             val useMachO = target.family.isAppleFamily
             val useElf = target.family in listOf(Family.LINUX, Family.ANDROID)

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)
     }
 }
-