crypto/fips140: add WithoutEnforcement

crypto/fips140: add WithoutEnforcement

WithoutEnforcement lets programs running under GODEBUG=fips140=only
selectively opt out of strict enforcement. This is especially helpful
for non-critical uses of cryptography routines like SHA-1 for content
addressable storage backends (E.g. git).

Fixes #74630

Change-Id: Iabba1f5eb63498db98047aca45e09c5dccf2fbdf
Reviewed-on: https://go-review.googlesource.com/c/go/+/723720
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>