Dynamically composite xml in reply to GDB

Dynamically composite xml in reply to GDB

GDBserver still uses pre-generated target descriptions in order to
reply to GDB's query on target description (see xml-builtin-generated.c
in GDBserver build directory).  This patch teaches GDBserver to
create XML contents according to the target descriptions rather than
using pre-generated ones.

First, change target feature c files to pass the feature xml file
name to tdesc_create_feature, so that target description in GDBserver
can record them, and create XML contents from these features in
buffer, like

  ...
  <xi:include href="$FEATURE1_XML_NAME"/>
  <xi:include href="$FEATURE2_XML_NAME"/>
  ...

and send this buffer back to GDB.

gdb/gdbserver:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* linux-x86-tdesc.c (i386_get_ipa_tdesc): Call
	set_tdesc_architecture and set_tdesc_osabi.  Don't set
	xmltarget field.
	* server.c (get_features_xml): Call tdesc_get_features_xml.
	* tdesc.c (set_tdesc_architecture): New function.
	(set_tdesc_osabi): New function.
	(tdesc_get_features_xml): New function.
	(tdesc_create_feature): Add one argument.
	* tdesc.h (struct target_desc) <features>: New field.
	<arch>: New field.
	<osabi>: New field.
	(target_desc::~target_desc): xfree arch, osabi, and
	features.
	(set_tdesc_architecture): Declare.
	(set_tdesc_osabi): Declare.
	(tdesc_get_features_xml): Declare.
	* target-descriptions.c (tdesc_create_feature): Add one
	argument.
	* target-descriptions.h (tdesc_create_feature): Update
	the declaration.