gas dwarf2dbg.c tidy

Make it a little more obvious that remap_debug_filename returns an
allocated string (that should be freed) by returning a char * rather
than const char *.  Free a few missed cases in dwarf2dbg.c, and free
other memory allocated in dwarf2dbg.c.  Also remove static
initialisation of variables and initialise in dwarf2_init instead,
in order to ensure gas state is saner for oss-fuzz.

	* remap.c (remap_debug_filename): Remove const from return.
	* as.h (remap_debug_filename): Update prototype.
	* config/obj-elf.c (obj_elf_ident): Simplify free of
	remap_debug_filename output.
	* stabs.c (stabs_generate_asm_file): Likewise.
	* dwarf2dbg.c (dirs, dirs_in_use, dirs_allocated, current): Don't
	initialise statically..
	(dwarf2_init): ..do so here, along with most other static vars.
	(assign_file_to_slot): Don't set files_allocated until we
	succeed in allocating memory.
	(purge_generated_debug): Add bool param, free more stuff if true.
	(dwarf2_directive_filename): Adjust purge_generated_debug call.
	(process_entries): Don't free line_entry here..
	(dwarf2_cleanup): ..do so here instead, new function.
	(dwarf2_finish): Call dwarf2_cleanup.  When chaining together
	subseg line entries, unhook entries from old subseg list.
	(dwarf2_directive_loc): Free remap_debug_filename string.
	(out_dir_and_file_list): Likewise.
	(out_debug_str): Likewise.

gas/as.h

@@ -487,7 +487,7 @@ void   register_dependency (const char *);
 void   print_dependencies (void);
 segT   subseg_get (const char *, int);
 
-const char *remap_debug_filename (const char *);
+char *remap_debug_filename (const char *);
 void add_debug_prefix_map (const char *);
 
 static inline char *