Move default implementation of TargetPlatform.platformName to leaf classes

Otherwise, we have a static initialization loop, leading to null-leaks

Removing default interface method indeed disconnects the loop, as per JVM
Specification, "5.5 Initialization".

See KT-33245 for detailed explanations

^KT-33245 Fixed

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsPlatform.kt

@@ -27,10 +27,6 @@ import org.jetbrains.kotlin.resolve.TargetPlatform
     level = DeprecationLevel.ERROR
 )
 interface JsPlatform : TargetPlatform {
-    @JvmDefault
-    override val platformName: String
-        get() = "JS"
-
     companion object {
         @JvmField
         val INSTANCE: JsPlatform = JsPlatforms.CompatJsPlatform

js/js.frontend/src/org/jetbrains/kotlin/platform/js/JsPlatform.kt

@@ -24,7 +24,10 @@ object JsPlatforms {
     )
     object CompatJsPlatform : TargetPlatform(setOf(DefaultSimpleJsPlatform)),
         // Needed for backward compatibility, because old code uses INSTANCEOF checks instead of calling extensions
-        org.jetbrains.kotlin.js.resolve.JsPlatform {}
+        org.jetbrains.kotlin.js.resolve.JsPlatform {
+        override val platformName: String
+            get() = "JS"
+    }
 
     val defaultJsPlatform: TargetPlatform
         get() = CompatJsPlatform