PPC64: Allow .TOC. in linker script to override backend calculated value

PPC64: Allow .TOC. in linker script to override backend calculated value

bfd/
	* elf64-ppc.c (ppc64_elf_func_desc_adjust): Don't redefine .TOC.
	if already defined, and set linker_def.
	(ppc64_elf_set_toc): Use .TOC. value if defined other than by
	the backend.
ld/
	* ldexp.c (exp_fold_tree_1): Clear linker_def on symbol assignment.