cmd/internal/obj/arm64: don't immediate dereference new prog

Noticed by Cherry while reviewing CL 40252.

The alternative to this is to place t on the stack, like

t := obj.Prog{Ctxt: ctxt}

However, there are only a couple of places where we
manually construct Progs, which is useful.

This isn't hot enough code to warrant
breaking abstraction layers to avoid an allocation.

Passes toolstash-check.

Change-Id: I46c79090b60641c90ee977b750ba5c708aca8ecf
Reviewed-on: https://go-review.googlesource.com/40373
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/arm64/asm7.go

@@ -732,7 +732,7 @@ func (c *ctxt7) flushpool(p *obj.Prog, skip int) {
 func (c *ctxt7) addpool(p *obj.Prog, a *obj.Addr) {
 	cls := c.aclass(a)
 	lit := c.instoffset
-	t := *c.newprog()
+	t := c.newprog()
 	t.As = AWORD
 	sz := 4
 
@@ -804,7 +804,7 @@ func (c *ctxt7) addpool(p *obj.Prog, a *obj.Addr) {
 	}
 
 	q := c.newprog()
-	*q = t
+	*q = *t
 	q.Pc = int64(c.pool.size)
 	if c.blitrl == nil {
 		c.blitrl = q