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/fips140enforce.go

@@ -0,0 +1,13 @@
+//go:build fips140enforce
+
+package noiseutil
+
+import (
+	"crypto/fips140"
+)
+
+func init() {
+	if !fips140.Enforced() {
+		panic("Nebula compiled with fips140 expects FIPS140 to be enforced. Do not set GODEBUG=fips140, or if you do it must be set as GODEBUG=fips140=only")
+	}
+}