jit: make gdb_object::symtabs an std::forward_list

jit: make gdb_object::symtabs an std::forward_list

Replace the manual linked list with an std::forward_list, simplifying
the memory management.  This requires allocating gdb_object with new and
free'ing it with delete.

gdb/ChangeLog:

	* jit.c: Include forward_list.
	(struct gdb_symtab) <next>: Remove field.
	(struct gdb_object) <symtabs>: Change type to
	std::forward_list<gdb_symtab>.
	(jit_object_open_impl): Allocate gdb_object with new.
	(jit_symtab_open_impl): Adjust to std::forward_list.
	(finalize_symtab): Don't delete symtab.
	(jit_object_close_impl):  Adjust to std::forward_list.  Free
	gdb_object with delete.