cmd/internal/obj: use REGENTRYTMP* in a few more places

There are a few remaining places in obj6 where we hard-code
safe-on-entry registers. Fix those to use the consts.

For #40724.

Change-Id: Ie640521aa67d6c99bc057553dc122160049c6edc
Reviewed-on: https://go-review.googlesource.com/c/go/+/307009


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>