Harmonize and improve auxiliary entries support for XCOFF

Harmonize and improve auxiliary entries support for XCOFF

bfd/ChangeLog:
	* coff-rs6000.c (_bfd_xcoff_swap_aux_in): Add errors for
	unsupported storage class or auxialiry entries.
	Improve and adapt to new aux structures.
	Add C_DWARF support.
	(_bfd_xcoff_swap_aux_out): Likewise.
	* coff64-rs6000.c (_bfd_xcoff64_swap_aux_in): Likewise.
	(_bfd_xcoff64_swap_aux_out): Likewise.

binutils/ChangeLog:
	* od-xcoff.c (dump_xcoff32_symbols): Adapt to new
	aux structures.

include/ChangeLog:
	* coff/internal.h (union internal_auxent):
	 Add x_sect structure.
	* coff/rs6000.h (union external_auxent): Rework to
	match official documentation.
	* coff/rs6k64.h (union external_auxent): Likewise.
	(_AUX_SECT): New define.