[RS6000] .gnu.attributes Tag_GNU_Power_ABI_FP

Extend this attribute to cover long double ABIs, for 64-bit too.

This patch also corrects an error that crept in to code setting
rs6000_passes_float.  See the added comment.  Passing IEEE128 values
in vsx regs ought to set both Tag_GNU_Power_ABI_FP and
Tag_GNU_Power_ABI_Vector.  Also adds a new option, default on, that
disables output of .gnu_attribute assembly directives.

	* config/rs6000/sysv4.opt (mgnu-attribute): New option.
	* doc/invoke.texi: Document it.
	* config/rs6000/rs6000.c (HAVE_LD_PPC_GNU_ATTR_LONG_DOUBLE): Define.
	(rs6000_passes_float): Comment.
	(rs6000_passes_long_double): New static var.
	(call_ABI_of_interest): Return false unless rs6000_gnu_attr is set.
	(init_cumulative_args): Set up to emit fp .gnu_attribute for
	ELF 64-bit ABIs as well as 32-bit ELF.  Correct rs6000_passes_float
	to include fp values returned in vectors.
	Set rs6000_passes_long_double.
	(rs6000_function_arg_advance_1): Likewise for function args.
	(rs6000_elf_file_end): Emit fp .gnu_attribute for ELF 64-bit ABIs,
	and SPE.  Emit long double tag value too.
	(rs6000_opt_vars): Add gnu-attr.
	* configure.ac (HAVE_LD_PPC_GNU_ATTR_LONG_DOUBLE): New ppc32 test.
	* configure: Regenerate.
	* config.in: Regenerate.

From-SVN: r240601