Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modernc.org/libc/builtin32.go

@@ -0,0 +1,29 @@
+// Copyright 2024 The Libc 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 linux && (386 || arm)
+
+package libc // import "modernc.org/libc"
+
+import (
+	mbits "math/bits"
+)
+
+func X__builtin_ctzl(tls *TLS, x ulong) int32 {
+	return int32(mbits.TrailingZeros32(x))
+}
+
+func X__builtin_clzl(t *TLS, n ulong) int32 {
+	return int32(mbits.LeadingZeros32(n))
+}
+
+// int __builtin_popcountl (unsigned long x)
+func X__builtin_popcountl(t *TLS, x ulong) int32 {
+	return int32(mbits.OnesCount32(x))
+}
+
+// int __builtin_popcountll (unsigned long long)
+func X__builtin_popcountll(t *TLS, x uint64) int32 {
+	return int32(mbits.OnesCount64(x))
+}