crypto/tls: align FIPS-only mode with BoringSSL policy

This enables TLS 1.3, disables P-521, and disables non-ECDHE suites.

Reapplies CL 549975.

Updates #64717
Updates #62372

Change-Id: I6c608704638d59a063a657fbd4eb1126027112dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/603376
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

handshake_server_tls13.go

@@ -64,10 +64,6 @@ func init() {
 func (hs *serverHandshakeStateTLS13) handshake() error {
 	c := hs.c
 
-	if needFIPS() {
-		return errors.New("tls: internal error: TLS 1.3 reached in FIPS mode")
-	}
-
 	// For an overview of the TLS 1.3 handshake, see RFC 8446, Section 2.
 	/*
 		if err := hs.processClientHello(); err != nil {
@@ -218,6 +214,9 @@ func (hs *serverHandshakeStateTLS13) processClientHello() error {
 	if !hasAESGCMHardwareSupport || !aesgcmPreferred(hs.clientHello.cipherSuites) {
 		preferenceList = defaultCipherSuitesTLS13NoAES
 	}
+	if needFIPS() {
+		preferenceList = defaultCipherSuitesTLS13FIPS
+	}
 	for _, suiteID := range preferenceList {
 		hs.suite = mutualCipherSuiteTLS13(hs.clientHello.cipherSuites, suiteID)
 		if hs.suite != nil {