math: make sqrt smaller on AMD64

This makes function fit in 16 bytes, saving 16 bytes.

Change-Id: Iac5d2add42f6dae985b2a5cbe19ad4bd4bcc92ec
Reviewed-on: https://go-review.googlesource.com/29151
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/math/sqrt_amd64.s

@@ -6,7 +6,7 @@
 
 // func Sqrt(x float64) float64
 TEXT ·Sqrt(SB), NOSPLIT, $0
-	MOVSD  x+0(FP), X0
-	SQRTSD X0, X1
-	MOVSD  X1, ret+8(FP)
+	XORPS  X0, X0 // break dependency
+	SQRTSD x+0(FP), X0
+	MOVSD  X0, ret+8(FP)
 	RET