Use std::vector and std::string instead of VEC(char_ptr) in gdbserver tdesc

Use std::vector and std::string instead of VEC(char_ptr) in gdbserver tdesc

This is a straightforward replacement, no change in behavior are
intended/expected.

gdb/gdbserver/ChangeLog:

	* tdesc.h (struct target_desc) <features>: Change type to
	std::vector<std::string>.
	* tdesc.c (target_desc::~target_desc): Adjust to std::vector
	changes.
	(tdesc_get_features_xml): Likewise.
	(tdesc_create_feature): Likewise.