crypto/internal/fips/subtle: provide XORBytes

crypto/internal/fips/subtle: provide XORBytes

This is needed from inside the module, and we generally don't want to
import the crypto tree from it.

For #69536

Change-Id: I69e91e4df89ecac0016c671ccd28e733a7131533
Reviewed-on: https://go-review.googlesource.com/c/go/+/616716
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>