objdump -g on gcc COFF/PE files

objdump -g can't be used much.  Trying to dump PE files invariably
seems to run into "debug_name_type: no current file" or similar
errors, because parse_coff expects a C_FILE symbol to be the first
symbol.  Dumping -gstabs output works since the N_SO stab is present.
Pre-setting the file name won't hurt stabs dumping.

	* rddbg.c (read_debugging_info): Call debug_set_filename.