math: Sqrt not intrinsified on mips64

[ALTree]

$ go version
go version go1.10 linux/amd64

func f(x float64) float64 {
	return math.Sqrt(x)
}

GOARCH=mips:

0x0004 00004 (test.go:5)	MOVF	"".x+4(FP), F0
0x0008 00008 (test.go:6)	SQRTD	F0, F0

but with GOARCH=mips64:

0x002c 00044 (test.go:5)	MOVD	"".x(FP), F0
0x0030 00048 (test.go:6)	MOVD	F0, 8(R29)
0x0034 00052 (test.go:6)	CALL	math.Sqrt(SB)

[gopherbot]

Change https://golang.org/cl/95916 mentions this issue: cmd/compile: intrinsify math.sqrt on mips64