Support signal mechanism for ch8(only works on signal-thread apps)

user/src/bin/infloop.rs

@@ -0,0 +1,10 @@
+#![no_std]
+#![no_main]
+#![allow(clippy::empty_loop)]
+
+extern crate user_lib;
+
+#[no_mangle]
+pub fn main(_argc: usize, _argv: &[&str]) -> ! {
+    loop {}
+}