build: update assembly variable names for vet

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7834046
diff --git a/src/pkg/math/sqrt_386.s b/src/pkg/math/sqrt_386.s
index d0a428d..824fa63 100644
--- a/src/pkg/math/sqrt_386.s
+++ b/src/pkg/math/sqrt_386.s
@@ -6,5 +6,5 @@
 TEXT ·Sqrt(SB),7,$0
 	FMOVD   x+0(FP),F0
 	FSQRT
-	FMOVDP  F0,r+8(FP)
+	FMOVDP  F0,ret+8(FP)
 	RET