cmd/go,crypto/internal/fips140: prevent using FIPS 140-3 mode with purego tag

cmd/go,crypto/internal/fips140: prevent using FIPS 140-3 mode with purego tag

Change-Id: I6a6a696414f8d5d9dc77c65b0ac9edfc982c2798
Reviewed-on: https://go-review.googlesource.com/c/go/+/703095
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>