Change plt stubs to have destination in r12.

Change plt stubs to have destination in r12.

This change is to support the new ELFv2 ABI, which uses the value in
r12 on function entry to calculate the got/toc pointer.

bfd/
	* elf64-ppc.c (build_plt_stub): Switch stubs to use r11 as base
	reg and r12 as destination.
	(ppc_build_one_stub): Likewise.
	(ppc64_elf_build_stubs): Likewise for glink.
ld/testsuite/
	* ld-powerpc/tls.s: Add proper .opd entry for _start.
	* ld-powerpc/tlstoc.s: Likewise.
	* ld-powerpc/relbrlt.d: Update for changed stubs.
	* ld-powerpc/tls.d: Update for changed stubs and _start .opd entry.
	* 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.d: Likewise.
	* ld-powerpc/tlstoc.g: Likewise.
	* ld-powerpc/tlstocso.d: Likewise.
	* ld-powerpc/tlstocso.g: Likewise.
	* ld-powerpc/tlstocso.r: Likewise.