powerpc relax_section and section contents cache

This patch provides a means for backend relax_section support to
increase the size of a section without needing to reallocate
section contents.  This helps reduce memory usage when the added space
does not need to be written in relax_section, as is the case for
powerpc.  Writing the stubs later means a few tweaks are needed in the
powerpc relocate_section function, but also removes some code
duplication since the extra ld -r relocs can be written there too.

	* elf-bfd.h (struct elf_backend_data): Add caches_rawsize.
	* elfxx-target.h (elf_backend_caches_rawsize): Define.
	(elfNN_bed): Init new field.
	* elflink.c (elf_link_input_bfd): Handle caches_rawsize.
	* elf32-ppc.c (shared_stub_entry): Zero addi offset.
	(ppc_elf_relax_section): Don't reallocate section here, write
	stubs, or write out relocs for ld -r here..
	(ppc_elf_relocate_section): ..instead write stubs here, and use
	existing code to write out relocs for ld -r.  Fix offset
	adjustment on reloc for little-endian.
	(elf_backend_caches_rawsize): Define.