cmd/compile: add newnamel, use in tempAt

cmd/compile: add newnamel, use in tempAt

newnamel is newname but with no dependency on lineno or Curfn.
This makes it suitable for use in a concurrent back end.
Use it now to make tempAt global-free.

The decision to push the assignment to n.Name.Curfn
to the caller of newnamel is based on mdempsky's
comments in #19683 that he'd like to do that
for callers of newname as well.

Passes toolstash-check. No compiler performance impact.

Updates #19683
Updates #15756

Change-Id: Idc461a1716916d268c9ff323129830d9a6e4a4d9
Reviewed-on: https://go-review.googlesource.com/39191
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>