salsa20: add loong64 SIMD implementation

The performance gains on Loongson 3A6000 and 3A5000 are as follows:

goos: linux
goarch: loong64
pkg: golang.org/x/crypto/salsa20
cpu: Loongson-3A6000-HV @ 2500.00MHz
      |  bench.old   |              bench.new              |
      |    sec/op    |   sec/op     vs base                |
XOR1K   3175.0n ± 0%   435.4n ± 0%  -86.29% (p=0.000 n=20)

      |  bench.old   |               bench.new                |
      |     B/s      |      B/s       vs base                 |
XOR1K   307.6Mi ± 0%   2242.7Mi ± 0%  +629.13% (p=0.000 n=20)

goos: linux
goarch: loong64
pkg: golang.org/x/crypto/salsa20
cpu: Loongson-3A5000 @ 2500.00MHz
      |  bench.old   |              bench.new              |
      |    sec/op    |   sec/op     vs base                |
XOR1K   4125.0n ± 0%   864.0n ± 0%  -79.05% (p=0.000 n=20)

      |  bench.old   |               bench.new                |
      |     B/s      |      B/s       vs base                 |
XOR1K   236.7Mi ± 0%   1130.3Mi ± 0%  +377.41% (p=0.000 n=20)

Change-Id: Ib37f603e6654f1e3837985fad4b6dee10b5af993
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/663375
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>

Author: sophie-zhao

salsa20/salsa/salsa20_loong64.go

@@ -0,0 +1,29 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build loong64 && !purego && gc
+
+package salsa
+
+import "golang.org/x/sys/cpu"
+
+// XORKeyStreamVX is implemented in salsa20_loong64.s.
+//
+//go:noescape
+func XORKeyStreamVX(out, in *byte, n uint64, nonce, key *byte)
+
+// XORKeyStream crypts bytes from in to out using the given key and counters.
+// In and out must overlap entirely or not at all. Counter
+// contains the raw salsa20 counter bytes (both nonce and block counter).
+func XORKeyStream(out, in []byte, counter *[16]byte, key *[32]byte) {
+	if len(in) == 0 {
+		return
+	}
+	_ = out[len(in)-1]
+	if cpu.Loong64.HasLSX {
+		XORKeyStreamVX(&out[0], &in[0], uint64(len(in)), &counter[0], &key[0])
+	} else {
+		genericXORKeyStream(out, in, counter, key)
+	}
+}