reloc caching

This arranges to free section relocs cached in elf_section_data.  To
do that, some relocs stored there need to use bfd_malloc buffers
rather than bfd_alloc ones.

	* elf.c (_bfd_elf_free_cached_info): Free relocs.
	* elf32-ppc.c (ppc_elf_relax_section): Realloc relocs rather
	than malloc, copy, free old.
	* elf64-ppc.c (get_relocs): bfd_malloc relocs.
	* elflink.c (_bfd_elf_link_info_read_relocs): Always
	bfd_malloc relocs.