Add PowerPC64 ld --tls-get-addr-optimize.

Add PowerPC64 ld --tls-get-addr-optimize.

Sometimes it may be of benefit to force use of the __tls_get_addr_opt
call stub even when the glibc being used during linking does not
advertise __tls_get_addr_opt.

bfd/
	* elf64-ppc.h (struct ppc64_elf_params <tls_get_addr_opt>): Rename
	from no_tls_get_addr_opt.
	* elf64-ppc.c: Update for rename and inversion of tls_get_addr_opt.
	(ppc64_elf_tls_setup): Set tls_get_addr_opt to 0 only when at
	default of -1.
ld/
	* emultempl/ppc64elf.em (params): Init tls_get_addr_opt field to -1.
	(OPTION_TLS_GET_ADDR_OPT): Define.
	(PARSE_AND_LIST_LONGOPTS): Handle --tls-get-addr-opt.
	(PARSE_AND_LIST_OPTIONS, PARSE_AND_LIST_ARGS_CASES): Likewise.
	* ld.texinfo: Document --tls-get-addr-optimize and
	--no-tls-get-addr-optimize.