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

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>