A couple of tests for default libraries.

backend.native/tests/build.gradle

@@ -1220,6 +1220,17 @@ task link(type: LinkKonanTest) {
 }
 */
 
+task link_omit_unused(type: LinkKonanTest) {
+    goldValue = "Hello\n"
+    source = "link/omit/hello.kt"
+    lib = "link/omit/lib.kt"
+}
+
+task link_default_libs(type: RunKonanTest) {
+    goldValue = "ptrdiff = 0\n"
+    source = "link/default/default.kt"
+}
+
 task for0(type: RunKonanTest) {
     goldValue = "2\n3\n4\n"
     source = "runtime/basic/for0.kt"

backend.native/tests/link/default/default.kt

@@ -0,0 +1,7 @@
+import posix.*
+
+fun main(args: Array<String>) {
+    // Just check the typealias is in scope.
+    val ptrdiff: ptrdiff_t = 0L
+    println("ptrdiff = $ptrdiff")
+}

backend.native/tests/link/omit/hello.kt

@@ -0,0 +1,3 @@
+fun main(args: Array<String>) {
+    println("Hello")
+}

backend.native/tests/link/omit/lib.kt

@@ -0,0 +1,2 @@
+package foo
+fun foo() {}