cmd/go: allow defaults for removed GODEBUG settings

With this change, removed GODEBUG settings may still be used by clients
provided that they don't use an old (non-final) value for the setting.

For instance, with Go 1.27, the gotypesalias setting will be removed.
It's old value was "0"; the new default behavior is indicated by any
non-"0" value (usually "1"). Programs that set //go:debug gotypesalias=1
(or the respective godebug gotypesalias=1 in a .mod file) and thus rely
on the new final behavior will keep building unchanged. Removing the
setting is encouraged but not required. But programs that use an old
setting (//go:debug gotypesalias=0 in this example) will fail to build.

While at it, add tlskyber to the set of removed GODEBUG settings
(was missed when it was removed/renamed - thanks to Mateusz Poliwczak
for noticing).

Add test cases verifying the new behavior.

Fixes #79422.
For #76163.

Change-Id: I0307fec7c0a5ce776cfdd7b95db9bd76295ba7f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/784221
Reviewed-by: Ian Alexander <jitsu@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>