[dev.power64] math: implement Abs, add power64/power64le forwarding assembly functions

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/119670043

src/pkg/math/abs_power64x.s

+// Copyright 2011 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 power64 power64le
+
+#include "../../cmd/ld/textflag.h"
+
+TEXT ·Abs(SB),NOSPLIT,$0-16
+	MOVD	x+0(FP), R3
+	MOVD 	$((1<<63)-1), R4
+	AND	R4, R3
+	MOVD	R3, ret+8(FP)
+	RETURN

src/pkg/math/stubs_power64x.s

+// Copyright 2014 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 power64 power64le
+
+#include "../../cmd/ld/textflag.h"
+
+TEXT ·Asin(SB),NOSPLIT,$0
+	BR ·asin(SB)
+
+TEXT ·Acos(SB),NOSPLIT,$0
+	BR ·acos(SB)
+
+TEXT ·Atan2(SB),NOSPLIT,$0
+	BR ·atan2(SB)
+
+TEXT ·Atan(SB),NOSPLIT,$0
+	BR ·atan(SB)
+
+TEXT ·Dim(SB),NOSPLIT,$0
+	BR ·dim(SB)
+
+TEXT ·Min(SB),NOSPLIT,$0
+	BR ·min(SB)
+
+TEXT ·Max(SB),NOSPLIT,$0
+	BR ·max(SB)
+
+TEXT ·Exp2(SB),NOSPLIT,$0
+	BR ·exp2(SB)
+
+TEXT ·Expm1(SB),NOSPLIT,$0
+	BR ·expm1(SB)
+
+TEXT ·Exp(SB),NOSPLIT,$0
+	BR ·exp(SB)
+
+TEXT ·Floor(SB),NOSPLIT,$0
+	BR ·floor(SB)
+
+TEXT ·Ceil(SB),NOSPLIT,$0
+	BR ·ceil(SB)
+
+TEXT ·Trunc(SB),NOSPLIT,$0
+	BR ·trunc(SB)
+
+TEXT ·Frexp(SB),NOSPLIT,$0
+	BR ·frexp(SB)
+
+TEXT ·Hypot(SB),NOSPLIT,$0
+	BR ·hypot(SB)
+
+TEXT ·Ldexp(SB),NOSPLIT,$0
+	BR ·ldexp(SB)
+
+TEXT ·Log10(SB),NOSPLIT,$0
+	BR ·log10(SB)
+
+TEXT ·Log2(SB),NOSPLIT,$0
+	BR ·log2(SB)
+
+TEXT ·Log1p(SB),NOSPLIT,$0
+	BR ·log1p(SB)
+
+TEXT ·Log(SB),NOSPLIT,$0
+	BR ·log(SB)
+
+TEXT ·Modf(SB),NOSPLIT,$0
+	BR ·modf(SB)
+
+TEXT ·Mod(SB),NOSPLIT,$0
+	BR ·mod(SB)
+
+TEXT ·Remainder(SB),NOSPLIT,$0
+	BR ·remainder(SB)
+
+TEXT ·Sincos(SB),NOSPLIT,$0
+	BR ·sincos(SB)
+
+TEXT ·Sin(SB),NOSPLIT,$0
+	BR ·sin(SB)
+
+TEXT ·Cos(SB),NOSPLIT,$0
+	BR ·cos(SB)
+
+TEXT ·Sqrt(SB),NOSPLIT,$0
+	BR ·sqrt(SB)
+
+TEXT ·Tan(SB),NOSPLIT,$0
+	BR ·tan(SB)