crypto/internal/fips140/mlkem: make CAST conditional

crypto/internal/fips140/mlkem: make CAST conditional

It taks north of 130µs on my machine, which is enough to be worth
shaving off at init time.

Change-Id: I6a6a696463de228bc3e7b9ca10c12ddbaabdf384
Reviewed-on: https://go-review.googlesource.com/c/go/+/707695
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>