jit: c++-ify gdb_symtab

jit: c++-ify gdb_symtab

This patch makes the gdb_symtab bit more c++y, in preparation for the
next patch that will use an std::forward_list<gdb_symtab>.  It changes
the fields to use automatic memory management, in the form of
std::string and gdb::unique_xmalloc_ptr, and adds a constructor and a
destructor.

gdb/ChangeLog:

	* jit.c (struct gdb_symtab): Add constructor, destructor,
	initialize fields.
	<linetable>: Change type to unique_xmalloc_ptr.
	<file_name>: Change type to std::string.
	(jit_symtab_open_impl): Allocate gdb_symtab with new.
	(jit_symtab_line_mapping_add_impl): Adjust.
	(finalize_symtab): Adjust, call delete on stab.