cmd/compile: hide testdclstack behind debug flag

This reduces compilation time for the program
in #18602 from 7 hours to 30 min.

Updates #14781
Updates #18602

Change-Id: I3c4af878a08920e6373d3b3b0c4453ee002e32eb
Reviewed-on: https://go-review.googlesource.com/35113


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>