Add sys_hello and testbin/hellotest

`sys_hello` is a new syscall that just prints some text in the terminal.

`testbin/hellotest` is a userland binary that just calls `sys_hello`

kern/syscall/hello.c

@@ -0,0 +1,7 @@
+#include <types.h>
+#include <lib.h>
+#include <syscall.h>
+int sys_hello(void) {
+  kprintf("Hello CSE4001!\n");
+  return 0;
+}