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/include/kern/syscall.h

@@ -94,6 +94,7 @@
 // #define SYS_getpriority 38
 // #define SYS_setpriority 39
 //                               (process groups, sessions, and job control)
+#define SYS_hello 41
 // #define SYS_getpgid    40
 // #define SYS_setpgid    41
 // #define SYS_getsid     42

kern/include/syscall.h

@@ -57,4 +57,6 @@ __DEAD void enter_new_process(int argc, userptr_t argv, userptr_t env,
 int sys_reboot(int code);
 int sys___time(userptr_t user_seconds, userptr_t user_nanoseconds);
 
+int sys_hello(void);
+
 #endif /* _SYSCALL_H_ */