[dev.regabi] cmd/compile: finish cleanup of Debug parsing

Now that the debug settings are in a struct, use struct tags to set
the usage messages and use reflection to populate debugtab,
much like we did for the Flag struct.

Change-Id: Id2ba30c30a9158c062527715a68bf4dd94679457
Reviewed-on: https://go-review.googlesource.com/c/go/+/272247


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>