Align .TOC. for PowerPC64

This change, with prerequisite 0e5fabeb, provides a toc base aligned
to 256 bytes rather than 8 bytes.  This is necessary for a minor gcc
optimisation, allowing use of d-form instructions to correctly access
toc-relative items larger than 8 bytes.

bfd/
	* elf64-ppc.c (TOC_BASE_ALIGN): Define.
	(ppc64_elf_next_toc_section): Align multi-got toc base.
	(ppc64_elf_set_toc): Likewise initial toc base and .TOC. symbol.
ld/
	* emulparams/elf64ppc.sh (GOT): Align.
ld/testsuite/
	* ld-powerpc/ambiguousv1b.d: Update for aligned .got.
	* ld-powerpc/defsym.d: Likewise.
	* ld-powerpc/elfv2-2exe.d: Likewise.
	* ld-powerpc/elfv2exe.d: Likewise.
	* ld-powerpc/elfv2so.d: Likewise.
	* ld-powerpc/relbrlt.d: Likewise.
	* ld-powerpc/tls.g: Likewise.
	* ld-powerpc/tlsexe.d: Likewise.
	* ld-powerpc/tlsexe.g: Likewise.
	* ld-powerpc/tlsexe.r: Likewise.
	* ld-powerpc/tlsexetoc.d: Likewise.
	* ld-powerpc/tlsexetoc.g: Likewise.
	* ld-powerpc/tlsexetoc.r: Likewise.
	* ld-powerpc/tlsso.d: Likewise.
	* ld-powerpc/tlsso.g: Likewise.
	* ld-powerpc/tlsso.r: Likewise.
	* ld-powerpc/tlstoc.g: Likewise.
	* ld-powerpc/tlstocso.d: Likewise.
	* ld-powerpc/tlstocso.g: Likewise.
	* ld-powerpc/tlstocso.r: Likewise.
	* ld-powerpc/tocopt.d: Likewise.
	* ld-powerpc/tocopt2.d: Likewise.
	* ld-powerpc/tocopt3.d: Likewise.
	* ld-powerpc/tocopt4.d: Likewise.
	* ld-powerpc/tocopt5.d: Likewise.