Use visitor pattern for "maint print c-tdesc"

Use visitor pattern for "maint print c-tdesc"

Target description can be modeled as a tree, the target description
is the root node, features are children nodes, registers and types are
grand-children nodes.  So command "maint print c-tdesc" in effect
traverse/visit each node, and print them in c.  This can be
implemented by visitor pattern, this is the first reason.  Secondly,
I want to this command prints c files in a different way for some
specific xml files, but still print c files the same way for the rest
of xml files.  Third, I even want to print xml files from target
descriptions, so that GDBserver can use it to reply GDB's query
qXfer:features:read:target.xml.

gdb:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* target-descriptions.c (tdesc_element_visitor): New class.
	(tdesc_element): New class.
	(tdesc_reg): Inherit from tdesc_element.
	(tdesc_reg::accept): New function.
	(tdesc_type): Inherit from tdesc_element.
	(tdesc_type::accept): New function.
	(tdesc_feature): Inherit from tdesc_element.
	(tdesc_feature::accept): New function.
	(target_desc): Inherit from tdesc_element.
	(target_desc::target_desc): New.
	(target_desc::~target_desc): New.
	(target_desc::accept): New.
	(allocate_target_description): Use new.
	(free_target_description): Use delete.
	(print_c_tdesc): New class.
	(maint_print_c_tdesc_cmd): Adjust.

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