Rename the tuning option and related functions to enable the Newton series for the...

Rename the tuning option and related functions to enable the Newton series for the reciprocal square root to reflect its approximative characteristic.

gcc/
	* config/aarch64/aarch64-protos.h (aarch64_emit_swrsqrt): Rename
	function to "aarch64_emit_approx_rsqrt".
	* config/aarch64/aarch64-tuning-flags.def: Rename tuning flag to
	AARCH64_EXTRA_TUNE_APPROX_RSQRT.
	* config/aarch64/aarch64.c (exynosm1_tunigs): Use new flag name.
	(xgene1_tunings): Likewise.
	(use_rsqrt_p): Likewise.
	(aarch64_emit_swrsqrt): Use new function name.
	* config/aarch64/aarch64-simd.md (aarch64_rsqrts_*): Likewise.
	* config/aarch64/aarch64.opt (mlow-precision-recip-sqrt): Reword the
	text explaining this option.
	* doc/invoke.texi (-mlow-precision-recip-sqrt): Likewise.

From-SVN: r233772