runtime: name hi and lo parts of ret in assembly

Makes vet happy.

LGTM=bradfitz
R=dvyukov, bradfitz
CC=golang-codereviews
https://golang.org/cl/131320043

src/pkg/runtime/asm_386.s

@@ -877,8 +877,8 @@ TEXT runtime·cputicks(SB),NOSPLIT,$0-4
 
 TEXT runtime·gocputicks(SB),NOSPLIT,$0-8
 	RDTSC
-	MOVL    AX, ret+0(FP)
-	MOVL    DX, ret+4(FP)
+	MOVL    AX, ret_lo+0(FP)
+	MOVL    DX, ret_hi+4(FP)
 	RET
 
 TEXT runtime·ldt0setup(SB),NOSPLIT,$16-0