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/conf/conf.kern

@@ -379,6 +379,7 @@ file      vfs/devnull.c
 file      syscall/loadelf.c
 file      syscall/runprogram.c
 file      syscall/time_syscalls.c
+file      syscall/hello.c
 
 #
 # Startup and initialization