[gdb/build] Fix static cast of virtual base

With this change in bfd/development.sh:
...
-development=true
+development=false
...
we run into:
...
In file included from tui-data.h:28:0,
                 from tui-command.c:24:
gdb-checked-static-cast.h: In instantiation of \
  ‘T gdb::checked_static_cast(V*) [with T = tui_cmd_window*; V = tui_win_info]’:
tui-command.c:65:15:   required from here
gdb-checked-static-cast.h:63:14: error: cannot convert from pointer to base \
  class ‘tui_win_info’ to pointer to derived class ‘tui_cmd_window’ because \
  the base is virtual
   T result = static_cast<T> (v);
              ^~~~~~~~~~~~~~~~~~
...

Fix this by using dynamic_cast instead of gdb::checked_static_cast in
TUI_CMD_WIN and TUI_STATUS_WIN.

Tested on x86_64-linux, with development set to false.

Reported-By: Robert Xiao <spam_hole@shaw.ca>
Reported-By: Simon Marchi <simark@simark.ca>
Approved-By: Tom Tromey <tom@tromey.com>

PR build/31399
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31399