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/bytecodeText/unsignedTypes/unsignedIntToString_jvm18.kt

@@ -0,0 +1,19 @@
+// JVM_TARGET: 1.8
+// WITH_RUNTIME
+// IGNORE_BACKEND: JVM_IR
+
+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"
+}
+
+// 0 kotlin/UInt.toString
+// 3 INVOKESTATIC java/lang/Integer.toUnsignedString \(I\)Ljava/lang/String;

compiler/testData/codegen/bytecodeText/unsignedTypes/unsignedLongToString_jvm18.kt

@@ -0,0 +1,19 @@
+// JVM_TARGET: 1.8
+// WITH_RUNTIME
+// IGNORE_BACKEND: JVM_IR
+
+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"
+}
+
+// 0 kotlin/ULong.toString
+// 3 INVOKESTATIC java/lang/Long.toUnsignedString \(J\)Ljava/lang/String;