Allow generation of nested target defined types in gdb target xml

Allow generation of nested target defined types in gdb target xml

This patch adds support to generate multiple nested architecture defined
data types in gdb target xml generated by openOCD. Architecture defined
structs, unions, vectors nested in one or more architecture defined
types can be generated now.

Example:

  <vector id="v2d" type="ieee_double" count="2"/>
  <vector id="v2u" type="uint64" count="2"/>
  <vector id="v2i" type="int64" count="2"/>

  <union id="vnd">
    <field name="f" type="v2d"/>
    <field name="u" type="v2u"/>
    <field name="s" type="v2i"/>
  </union>

Change-Id: I0f3c5c6daf3d22cde7e4b7b4165d2e97e25872f7
Signed-off-by: Omair Javaid <omair.javaid@linaro.org>
Reviewed-on: http://openocd.zylin.com/4372
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>