Control powerpc64-ld provision of register save/restore

This patch allows the user to override powerpc64-ld's default for
providing linker generated register save and restore functions as used
by gcc -Os code.  Normally these are not provided by ld -r, so Linux
kernel modules have needed to include their own copies.

bfd/
	* elf64-ppc.h (struct ppc64_elf_params): Add save_restore_funcs.
	* elf64-ppc.c (ppc64_elf_func_desc_adjust): Use it to control
	provision of out-of-line register save/restore routines.
ld/
	* emultempl/ppc64elf.em (params): Init new field.
	(ppc_create_output_section_statements): Set params.save_restore_funcs
	default.
	(PARSE_AND_LIST_*): Add support for --save-restore-funcs and
	--no-save-restore-funcs.