cmd/compile: change iexportVersionGeneric to 2

Don't expect/allow generics-related info in iexportVersion 1, now that
we increased the export version to 2.

Fixes #49853

Change-Id: I9bacee7f8e7cb9bb3b02a00084fad77edd220121
Reviewed-on: https://go-review.googlesource.com/c/go/+/367634


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>