Add ext.jvmTarget and ext.javaHome to core:descriptors.runtime

After the following changes:
* 290aded94f
* 33de71f792

default values of jvmTarget and javaHome started to overwrite properties
set through compileJava in core:descriptors.runtime

core/descriptors.runtime/build.gradle.kts

@@ -24,15 +24,8 @@ sourceSets {
     "test" { projectDefault() }
 }
 
-val compileJava by tasks.getting(JavaCompile::class) {
-    sourceCompatibility = "1.6"
-    targetCompatibility = "1.6"
-}
-
-val compileKotlin by tasks.getting(KotlinCompile::class) {
-    kotlinOptions.jvmTarget = "1.6"
-    kotlinOptions.jdkHome = rootProject.extra["JDK_16"] as String
-}
+jvmTarget = "1.6"
+javaHome = rootProject.extra["JDK_16"] as String
 
 val generateTests by generator("org.jetbrains.kotlin.generators.tests.GenerateRuntimeDescriptorTestsKt")