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.

interface.go

@@ -2,6 +2,7 @@ package nebula
 
 import (
 	"context"
+	"crypto/fips140"
 	"errors"
 	"fmt"
 	"log/slog"
@@ -270,6 +271,9 @@ func (f *Interface) activate() error {
 		"build", f.version,
 		"udpAddr", addr,
 		"boringcrypto", boringEnabled(),
+		"fips140Version", fips140.Version(),
+		"fips140Enabled", fips140.Enabled(),
+		"fips140Enforced", fips140.Enforced(),
 	)
 
 	if f.routines > 1 && !f.outside.SupportsMultipleReaders() {