Set emacs default mode for the GDB directory to C++

Set emacs default mode for the GDB directory to C++

Since GDB has switched to C++ but the file names are still .c emacs does
not load the proper mode when opening files in the gdb directory.

This patch fixes that by enabling c++ mode.

This patch also fixes indentation tweaks as discussed in this thread:
https://sourceware.org/ml/gdb-patches/2016-12/msg00074.html

Indent with gdb-code-style.el included and the .dir-locals.el is as such:

namespace TestNameSpace {

class test
{
public:
  test test() {}

  int m_a;
};

struct teststruct
{
  int a;
}
}

gdb/ChangeLog:

	* .dir-locals.el: Set c++ mode for the directory and set indent
	properly.
	* gdb-code-style.el: Set c-set-offset 'innamespace as a safe value
	to be used in .dir-locals.el.