cmd/pack: r command create output file if not exist

Go 1.15 pack's r command creates the output file if it does not
exist. The system "ar" command does this as well. Do the same.

For bazelbuild/rules_go#2762.

Change-Id: Icd88396b5c714b735c859a29ab29851e4301f4d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/279516


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>