[dev.power64] runtime: fix a syntax error that slipped in to asm_power64x.s

Apparently I had already moved on to fixing another problem
when I submitted CL 169790043.

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/165210043

src/runtime/asm_power64x.s

@@ -711,7 +711,7 @@ test:
 	CMP	R6, R7
 	BEQ	loop
 
-	MOVB	$0, ret+24(FP)
+	MOVB	R0, ret+24(FP)
 	RETURN
 
 // eqstring tests whether two strings are equal.