Make gdbserver reg_defs a vector of objects

Make gdbserver reg_defs a vector of objects

gdb/
	* regformats/regdef.h (reg): Add constructors.

gdb/gdbserver/
	* regcache.c (find_register_by_number): Return a ref.
	(find_regno): Use references.
	(register_size): Likewise.
	(register_data): Likewise.
	* tdesc.c (target_desc::~target_desc): Remove free calls.
	(target_desc::operator==): Use std::vector compare.
	(init_target_desc): Use reference.
	(tdesc_create_reg): Use reg constructors.
	* tdesc.h (struct target_desc): Replace pointer with object.