[AArch64] Add more choices for the reciprocal square root approximation

[AArch64] Add more choices for the reciprocal square root approximation

Allow a target to prefer such operation depending on the operation mode.

gcc/
	* config/aarch64/aarch64-protos.h
	(AARCH64_APPROX_MODE): New macro.
	(AARCH64_APPROX_{NONE,ALL}): Likewise.
	(cpu_approx_modes): New structure.
	(tune_params): New member "approx_modes".
	* config/aarch64/aarch64-tuning-flags.def
	(AARCH64_EXTRA_TUNE_APPROX_RSQRT): Remove macro.
	* config/aarch64/aarch64.c
	({generic,exynosm1,xgene1}_approx_modes): New core
	"cpu_approx_modes" structures.
	(generic_tunings): New member "approx_modes".
	(cortexa35_tunings): Likewise.
	(cortexa53_tunings): Likewise.
	(cortexa57_tunings): Likewise.
	(cortexa72_tunings): Likewise.
	(exynosm1_tunings): Likewise.
	(thunderx_tunings): Likewise.
	(xgene1_tunings): Likewise.
	(use_rsqrt_p): New argument for the mode and use new member from
	"tune_params".
	(aarch64_builtin_reciprocal): Devise mode from builtin.
	(aarch64_optab_supported_p): New argument for the mode.
	* doc/invoke.texi (-mlow-precision-recip-sqrt): Reword description.

From-SVN: r237395