gdb: additional settings for emacs in .dir-locals.el

Two additional settings for developers who use emacs:

  1. Set brace-list-open to 0 for C and C++ modes, this ensures we
  format things like:

  enum blah
  {
    ....
  };

  Instead of the default for the emacs GNU style:

  enum blah
    {
      ...
    };

  The former seems to be the GDB style.

  2. Set sentence-end-double-space to t.  This is actually the default
  value for this setting, but if anyone has customised this to nil in
  general, then forcing this back to t for GDB files will give a
  better behaviour for the paragraph filling.

gdb/ChangeLog:

	* .dir-locals.el: Set sentence-end-double-space for all modes, and
	set brace-list-open to 0 for C and C++ modes.

gdbserver/ChangeLog:

	* .dir-locals.el: Set sentence-end-double-space for all modes, and
	set brace-list-open to 0 for C and C++ modes.

gdbsupport/ChangeLog:

	* .dir-locals.el: Set sentence-end-double-space for all modes, and
	set brace-list-open to 0 for C and C++ modes.