Refactor gradle option generation mechanism

Ensure there's a statically checked dependency on LanguageVersion and
JvmTarget, so that this information is updated automatically once a new
language version or a JVM target is added

compiler/util/src/org/jetbrains/kotlin/config/JvmTarget.kt

@@ -22,6 +22,9 @@ enum class JvmTarget(val string: String) {
     ;
 
     companion object {
+        @JvmField
+        val DEFAULT = JVM_1_6
+
         @JvmStatic
         fun fromString(string: String) = values().find { it.string == string }
     }