Class-fy target_desc

This patch adds ctor and dtor in target_desc.

gdb:

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

	* target-descriptions.c (target_desc): Add ctor and dtor.  Do
	in-class initialization.
	(tdesc_create_feature): Call new instead of XCNEW.
	(free_target_description): Ue delete.