cmd/go,crypto: reject using Go+BoringCrypto and fips140 together

cmd/go,crypto: reject using Go+BoringCrypto and fips140 together

The combination is untested and nonsensical. Both are solutions to the
same problem.

For #69536

Change-Id: I95cc3baaf03b64ce08096e304e311a29e9577385
Reviewed-on: https://go-review.googlesource.com/c/go/+/637177
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>