gdb/emacs/dir-locals: Update settings for c++-mode

gdb/emacs/dir-locals: Update settings for c++-mode

The current .dir-locals file for GDB causes files that would usually
open in c-mode (for example, files ending in .c) to open in c++-mode.
However, all of the other settings applied for c-mode appear to get
reset when the file is switched over to c++-mode.

For example, we currently say:

 (c-mode . ((c-file-style . "GNU")
	    (mode . c++)
	    (indent-tabs-mode . t)
	    (tab-width . 8)
	    (c-basic-offset . 2)
	    (eval . (c-set-offset 'innamespace 0))
	    ))
 (c++-mode . ((eval . (when (fboundp 'c-toggle-comment-style)
			(c-toggle-comment-style 1)))))

So, when we enter c++-mode `indent-tabs-mode` is reset to its global
value, as are all of the other settings listed for c-mode.

This commit copies all of the settings (except the `mode` setting)
from the c-mode list to the c++-mode list.

The emacs documentation doesn't mention that `mode` causes this
resetting behaviour, so, in case this is an emacs bug, I'm using emacs
version 26.1.  Having the settings duplicated shouldn't cause any
problems except for a slight maintenance overhead.

gdb/ChangeLog:

	* .dir-locals.el: Copy most of the settings from c-mode over to
	c++-mode.