runtime: use DC ZVA instead of its encoding in WORD in arm64 memclr

Change-Id: I900a96b985f4d9378d25b9256c4274610fc2f70e
Reviewed-on: https://go-review.googlesource.com/c/go/+/698417
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: vleonen

src/runtime/memclr_arm64.s

@@ -176,7 +176,7 @@ aligned:
 
 	PCALIGN	$16
 loop_zva:
-	WORD	$0xd50b7420 // DC ZVA, R0
+	DC	ZVA, R0
 	ADD	R5, R0, R0
 	SUBS	R5, R1, R1
 	BHS	loop_zva