Fix display of structures/bitfields in register description.

Add support for displaying structures and bitfields for registers when
executing "maint print c-tdesc". This command is also used when
converting the xml target description file into c file.

Example of the behaviour is given below reporting a snipet of the xml file
and a snippet of the c code generated.

XML file contains:
...
    <union id="vecint">
      <field name="v4" type="v4int8"/>
      <field name="v2" type="v2int16"/>
    </union>

    <struct id="struct1">
      <field name="v4" type="v4int8"/>
      <field name="v2" type="v2int16"/>
    </struct>

    <struct id="struct2" size="8">
      <field name="f1" start="0" end="34"/>
      <field name="f2" start="63" end="63"/>
    </struct>
...

Setting this xml file as target description file and
issuing the maintenance print c-tdesc the following output
is obtained:

  feature = tdesc_create_feature (result, "extra");
  field_type = tdesc_named_type (feature, "int8");
  tdesc_create_vector (feature, "v4int8", field_type, 4);

  field_type = tdesc_named_type (feature, "int16");
  tdesc_create_vector (feature, "v2int16", field_type, 2);

  type = tdesc_create_union (feature, "vecint");
  field_type = tdesc_named_type (feature, "v4int8");
  tdesc_add_field (type, "v4", field_type);
  field_type = tdesc_named_type (feature, "v2int16");
  tdesc_add_field (type, "v2", field_type);

C output is not supported type "struct1".

This is finally the issue.

2013-03-27  Walfred Tedeschi  <walfred.tedeschi@intel.com>

        * target-descriptions.c (maint_print_c_tdesc_cmd):
        Add case to parse structures as register types and
        bitfields.

testsuite/

	* gdb.xml/maint_print_struct.exp: New file.
	* gdb.xml/maint_print_struct.xml: New file.

Change-Id: I2e20b095d508319c80275e724a9452c7e2834067
Signed-off-by: Walfred Tedeschi <walfred.tedeschi@intel.com>