[dev.typeparams] cmd/compile: use old export format if not compiling with generics

Write out export data with the old export format (iexportVersionPosCol)
if not compiling with generics (-G=0, the default value). This helps
ensure we don't break tests involving x/tools/go/gcexportdata (since we
can't modify that tool yet to use the new format).

Change-Id: I5f9bce44ed1e0696fc65fead6bab9e30de88461d
Reviewed-on: https://go-review.googlesource.com/c/go/+/323189


Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>