stdlib: Implement toString with radix in accordance with Kotlin JVM

Old Int.toString(radix: Int) and Long.toString(radix: Long)
implementations didn't add sign for negative values. E.g.
-1.toString(16) == "0xff..ff" while JVM returns "-1" in such case.
This patch fixes this behaviour and allow us to use any radix between
2 and 36 as JVM does.

backend.native/tests/runtime/text/to_string0.kt

@@ -0,0 +1,41 @@
+// Based on Apache Harmony tests.
+
+fun assertEquals(actual: String, expected: String, msg: String) {
+    if (actual != expected) throw AssertionError("$msg. Actual: $actual. Expected: $expected")
+}
+
+fun testIntToStringWithRadix() {
+    assertEquals(2147483647.toString(8),  "17777777777", "Octal string")
+    assertEquals(2147483647.toString(16), "7fffffff", "Hex string")
+    assertEquals(2147483647.toString(2),  "1111111111111111111111111111111", "Binary string")
+    assertEquals(2147483647.toString(10), "2147483647", "Decimal string")
+
+    assertEquals((-2147483647).toString(8),  "-17777777777", "Octal string")
+    assertEquals((-2147483647).toString(16), "-7fffffff", "Hex string")
+    assertEquals((-2147483647).toString(2),  "-1111111111111111111111111111111", "Binary string")
+    assertEquals((-2147483647).toString(10), "-2147483647", "Decimal string")
+
+    assertEquals((-2147483648).toString(8),  "-20000000000", "Octal string")
+    assertEquals((-2147483648).toString(16), "-80000000", "Hex string")
+    assertEquals((-2147483648).toString(2),  "-10000000000000000000000000000000", "Binary string")
+    assertEquals((-2147483648).toString(10), "-2147483648", "Decimal string")
+}
+
+fun testLongToStringWithRadix() {
+    assertEquals(100000000L.toString(10), "100000000", "Decimal string")
+    assertEquals(68719476735L.toString(16), "fffffffff", "Hex string")
+    assertEquals(8589934591L.toString(8), "77777777777", "Octal string")
+    assertEquals(8796093022207L.toString(2), "1111111111111111111111111111111111111111111", "Binary string")
+
+    assertEquals((-0x7fffffffffffffffL - 1).toString(10), "-9223372036854775808", "Min decimal string")
+    assertEquals(0x7fffffffffffffffL.toString(10), "9223372036854775807", "Max decimal string")
+    assertEquals((-0x7fffffffffffffffL - 1).toString(16), "-8000000000000000", "Min hex string")
+    assertEquals(0x7fffffffffffffffL.toString(16), "7fffffffffffffff", "Max hex string")
+
+}
+
+fun main(args: Array<String>) {
+    testIntToStringWithRadix()
+    testLongToStringWithRadix()
+    println("OK")
+}