dbg: update help for the lj-arch command

This patch adds the description of the LJ_DUALNUM dumped flag to the
lj-arch command's help.

Reviewed-by: Evgeniy Temirgaleev <e.temirgaleev@tarantool.org>
Reviewed-by: Sergey Bronnikov <sergeyb@tarantool.org>
Signed-off-by: Sergey Kaplun <skaplun@tarantool.org>
(cherry picked from commit 3e289b2)

Author: Buristan

src/luajit_dbg.py

@@ -1100,7 +1100,8 @@ class LJDumpArch(dbg.LJBase):
 
 The command requires no args and dumps values of LJ_64 and LJ_GC64
 compile-time flags. These values define the sizes of host and GC
-pointers, respectively.
+pointers, respectively. Also, it dumps the value for the LJ_DUALNUM
+compile-time flag to inspect if LuaJIT is built in dual-number mode.
     '''
 
     def execute(self, arg):