Support platform modifier in back-ends, add multi-platform integration test

Skip 'platform' declarations completely in back-ends

compiler/testData/multiplatform/simple/common.kt

@@ -0,0 +1,8 @@
+platform class Printer {
+    fun print(message: String)
+}
+
+fun main(args: Array<String>) {
+    val printer = Printer()
+    printer.print("Hello, world!")
+}

compiler/testData/multiplatform/simple/js.kt

@@ -0,0 +1,5 @@
+impl class Printer {
+    fun print(message: String) {
+        println("JS says: " + message)
+    }
+}

compiler/testData/multiplatform/simple/jvm.kt

@@ -0,0 +1,5 @@
+impl class Printer {
+    fun print(message: String) {
+        println("JVM says: " + message)
+    }
+}

compiler/testData/multiplatform/simple/output.txt

@@ -0,0 +1,11 @@
+-- Common --
+Exit code: OK
+Output:
+
+-- JVM --
+Exit code: OK
+Output:
+
+-- JS --
+Exit code: OK
+Output: