cmd/internal/obj: use REGENTRYTMP* in a few more places
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 <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go