Use bfd_get_filename throughout gdb

This patch makes gdb use the inline accessor for all bfd->filename
read accesses.

	* coff-pe-read.c (read_pe_exported_syms): Use bfd_get_filename
	rather than accessing bfd->filename directly.
	* dtrace-probe.c (dtrace_static_probe_ops::get_probes): Likewise,
	and use bfd_section_name.
	* dwarf2/frame.c (decode_frame_entry): Likewise.
	* exec.c (exec_set_section_address): Likewise.
	* solib-aix.c (solib_aix_bfd_open): Likewise.
	* stap-probe.c (get_stap_base_address): Likewise.
	* symfile.c (reread_symbols): Likewise.