Support native Golang "fips140" mode (#1696)

Add support for the "fips140" mode of Go:

- https://go.dev/doc/security/fips140
- https://csrc.nist.gov/projects/cryptographic-module-validation-program/certificate/5247

You can build with `make fips140`, see the README changes for more info.

Some differences from the boringcrypto builds:

- We switch to using `go:linkname crypto/tls.aeadAESGCMTLS13`, which gives us the fips implementation for both `boringcrypto` and `fips140` modes. This means we also no longer need `-checklinkname=0`
- Go native `fips140` doesn't need CGO_ENABLED=1
- We decide if we should use the fips140 GCM at runtime, if `fips140.Enabled()` is true. If you use the `make release-fips140`, we build with build tag `fips140enforce` which ensures the binary is running with fips140 enabled and that only P256 / AES-GCM is being used. If you don't want this enforce mode, you can build without the build tag.

cmd/nebula-cert/keygen.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"crypto/fips140"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -24,7 +26,7 @@ func newKeygenFlags() *keygenFlags {
 	cf.set.Usage = func() {}
 	cf.outPubPath = cf.set.String("out-pub", "", "Required: path to write the public key to")
 	cf.outKeyPath = cf.set.String("out-key", "", "Required: path to write the private key to")
-	cf.curve = cf.set.String("curve", "25519", "ECDH Curve (25519, P256)")
+	cf.curve = cf.set.String("curve", defaultCurve(), "ECDH Curve (25519, P256)")
 	cf.p11url = p11Flag(cf.set)
 	return &cf
 }
@@ -61,6 +63,9 @@ func keygen(args []string, out io.Writer, errOut io.Writer) error {
 	} else {
 		switch *cf.curve {
 		case "25519", "X25519", "Curve25519", "CURVE25519":
+			if fips140.Enforced() {
+				return errors.New("use of Curve25519 is not allowed in FIPS 140-only mode")
+			}
 			pub, rawPriv = x25519Keypair()
 			curve = cert.Curve_CURVE25519
 		case "P256":