[kernel] Add object_signal system call

Add a system call to assert signals on a given object, only within the
range of user-settable signals. Also made object_wait return
immediately if any of the given signals are already set.

src/kernel/syscalls.inc

@@ -2,6 +2,7 @@ SYSCALL(0x00, system_log,       const char *)
 SYSCALL(0x01, system_noop,      void)
 
 SYSCALL(0x09, object_wait,      j6_handle_t, j6_signal_t, j6_signal_t *)
+SYSCALL(0x0a, object_signal,    j6_handle_t, j6_signal_t)
 
 SYSCALL(0x10, process_koid,     j6_koid_t *)
 SYSCALL(0x11, process_exit,     int64_t)