crypto/internal/fips: disable CASTs if FIPS mode is not enabled

crypto/internal/fips: disable CASTs if FIPS mode is not enabled

Change-Id: Idabfe29e16d9ae6da7fbb078f9738bb4a7c5347b
Reviewed-on: https://go-review.googlesource.com/c/go/+/626935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>