Fix -Xmulti-platform dropped from args when freeCompilerArgs is assigned

in user build script

Issue #KT-15371 Fixed

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinMultiplatformPlugin.kt

@@ -54,10 +54,6 @@ open class KotlinPlatformImplementationPluginBase(platformName: String) : Kotlin
     private val platformKotlinTasksBySourceSetName = hashMapOf<String, AbstractKotlinCompile<*>>()
 
     override fun apply(project: Project) {
-        project.tasks.withType(AbstractKotlinCompile::class.java).all {
-            (it as KotlinCompile<*>).kotlinOptions.freeCompilerArgs += listOf("-Xmulti-platform")
-        }
-
         project.tasks.filterIsInstance<AbstractKotlinCompile<*>>().associateByTo(platformKotlinTasksBySourceSetName) { it.sourceSetName }
 
         val implementConfig = project.configurations.create(IMPLEMENT_CONFIG_NAME)

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -233,6 +233,8 @@ abstract class AbstractKotlinCompile<T : CommonCompilerArguments>() : AbstractKo
             args.verbose = true
         }
 
+        args.multiPlatform = project.plugins.any { it is KotlinPlatformPluginBase }
+
         setupPlugins(args)
     }