Add -Xsuppress-deprecated-jvm-target-warning to modules compiled with 1.6

Currently this leads to an unknown argument warning, but it'll be
removed automatically on the next bootstrap.

compiler/tests-common-jvm6/build.gradle.kts

@@ -1,4 +1,3 @@
-
 plugins {
     kotlin("jvm")
     id("jps-compatible")
@@ -17,3 +16,9 @@ sourceSets {
 }
 
 testsJar {}
+
+tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {
+    kotlinOptions {
+        freeCompilerArgs += "-Xsuppress-deprecated-jvm-target-warning"
+    }
+}