Handle backspace for k210/qemu platform && Add lots of user tests.

user/src/bin/stack_overflow.rs

@@ -0,0 +1,17 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+fn f(d: usize) {
+    println!("d = {}",d);
+    f(d + 1);
+}
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("It should trigger segmentation fault!");
+    f(0);
+    0
+}