Fix/suppress some warnings

(cherry picked from commit d8a43c216925b3a9e1475b786978436835a57927)

kotlin-native/backend.native/build.gradle

@@ -252,7 +252,7 @@ targetList.each { target ->
         def testCommon = project(":kotlin-test:kotlin-test-common").files("src/main/kotlin").files
         args = [*konanArgs,
                 '-output', project(':kotlin-native:runtime').file("build/${target}Stdlib"),
-                '-produce', 'library', '-module-name', 'stdlib', '-XXLanguage:+AllowContractsForCustomFunctions',
+                '-produce', 'library', '-module-name', 'stdlib',
                 '-Xmulti-platform', '-Xopt-in=kotlin.RequiresOptIn', '-Xinline-classes',
                 '-Xopt-in=kotlin.contracts.ExperimentalContracts',
                 '-Xopt-in=kotlin.ExperimentalMultiplatform',
@@ -358,4 +358,4 @@ publishing {
             from components.java
         }
     }
-}
+}