Introduce unspecifiedJvmPlatform for clients without jvmTarget

Previously, a lot of clients used JvmPlatform as platform-marker,
without thinking about jvmTarget.

For the sake of migration, this commits introduced so-called
UnspecifiedJvmPlatform, which can be used for a time being, but
generally, all usages should be removed in  future.

compiler/tests-common/tests/org/jetbrains/kotlin/resolve/lazy/lazyResolveTestUtils.kt

@@ -67,7 +67,7 @@ private class TestModule(val dependsOnBuiltIns: Boolean) : ModuleInfo {
                 ModuleInfo.DependencyOnBuiltIns.NONE
 
     override val platform: TargetPlatform
-        get() = JvmPlatforms.defaultJvmPlatform
+        get() = JvmPlatforms.unspecifiedJvmPlatform
 
     override val analyzerServices: PlatformDependentAnalyzerServices
         get() = JvmPlatformAnalyzerServices