math: implement asm modf for ppc64x

This change adds an asm implementations modf for ppc64x.

Improvements:

BenchmarkModf-16               7.48          6.26          -16.31%

Updates: #21390

Change-Id: I9c4f3213688e3e8842d050840dc04fc9c0bf6ce4
Reviewed-on: https://go-review.googlesource.com/74411
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>

src/math/modf_ppc64x.s

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ppc64 ppc64le
+
+#include "textflag.h"
+
+// func Modf(f float64) (int float64, frac float64)
+TEXT ·Modf(SB),NOSPLIT,$0
+	FMOVD	f+0(FP), F0
+	FRIZ	F0, F1
+	FMOVD	F1, int+8(FP)
+	FSUB	F1, F0, F2
+	FCPSGN	F2, F0, F2
+	FMOVD	F2, frac+16(FP)
+	RET

src/math/stubs_ppc64x.s

@@ -72,9 +72,6 @@ TEXT ·Log1p(SB),NOSPLIT,$0
 TEXT ·Log(SB),NOSPLIT,$0
 	BR ·log(SB)
 
-TEXT ·Modf(SB),NOSPLIT,$0
-	BR ·modf(SB)
-
 TEXT ·Mod(SB),NOSPLIT,$0
 	BR ·mod(SB)