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.

noiseutil/notboring.go

@@ -1,14 +1,25 @@
 //go:build !boringcrypto
-// +build !boringcrypto
 
 package noiseutil
 
 import (
+	"crypto/fips140"
+
 	"github.com/flynn/noise"
 )
 
 // EncryptLockNeeded indicates if calls to Encrypt need a lock
-const EncryptLockNeeded = false
+var EncryptLockNeeded = fips140.Enabled()
 
-// CipherAESGCM is the standard noise.CipherAESGCM when boringcrypto is not enabled
-var CipherAESGCM noise.CipherFunc = noise.CipherAESGCM
+var CipherAESGCM noise.CipherFunc = initAESGCM()
+
+func initAESGCM() noise.CipherFunc {
+	if fips140.Enabled() {
+		return CipherAESGCMFIPS140
+	} else {
+		return noise.CipherAESGCM
+	}
+
+}
+
+var boringEnabled = false