Stop the BFD library from automatically converting OS and PROC specific symbol sectio...

Stop the BFD library from automatically converting OS and PROC specific symbol section indicies to SHN_ABS, and provide a hook for backends to decide how such indicies should be processed.

	* elf-bfd.h (struct elf_backend_data): Add symbol_section_index
	callback.
	* elfxx-target.h (elf_backend_symbol_section_index): Provide
	default definition.
	(elfNN_bed): Initialise the symbol_section_index field.
	* elf.c (swap_out_syms): Call symbol_section_index, if defined, on
	OS and PROC specific section indicies.  Warn if converting other
	reserved incidies to SHN_ABS.