internal/godebugs: add fips140 as an opaque godebug setting
This permits using "godebug fips140=on" in go.mod and
using "//go:debug fips140=on" in the main package.
Change code references to the godebug setting to remove the #
which is no longer required.
For #71666
Change-Id: I3a60ecc55b03848dadd6d431eb43137b6df6568b
Reviewed-on: https://go-review.googlesource.com/c/go/+/649495 Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Filippo Valsorda <filippo@golang.org>