Add intrinsic for ULong/UInt.toString on Java 8+

Unfortunately this cannot currently be done for the extension overload which accepts a radix due to behavior difference with regard to invalid radix values.

compiler/testData/codegen/box/unsignedTypes/unsignedIntToString.kt

@@ -0,0 +1,15 @@
+// KJS_WITH_FULL_RUNTIME
+// WITH_RUNTIME
+
+fun box(): String {
+    val min = 0U.toString()
+    if ("0" != min) throw AssertionError(min)
+
+    val middle = 2_147_483_647U.toString()
+    if ("2147483647" != middle) throw AssertionError(middle)
+
+    val max = 4_294_967_295U.toString()
+    if ("4294967295" != max) throw AssertionError(max)
+
+    return "OK"
+}

compiler/testData/codegen/box/unsignedTypes/unsignedLongToString.kt

@@ -0,0 +1,15 @@
+// KJS_WITH_FULL_RUNTIME
+// WITH_RUNTIME
+
+fun box(): String {
+    val min = 0UL.toString()
+    if ("0" != min) throw AssertionError(min)
+
+    val middle = 9_223_372_036_854_775_807UL.toString()
+    if ("9223372036854775807" != middle) throw AssertionError(middle)
+
+    val max = 18_446_744_073_709_551_615UL.toString()
+    if ("18446744073709551615" != max) throw AssertionError(max)
+
+    return "OK"
+}