Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

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

vendor/golang.org/x/sys/cpu/cpu_arm64.s

@@ -0,0 +1,35 @@
+// Copyright 2019 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 gc
+
+#include "textflag.h"
+
+// func getisar0() uint64
+TEXT ·getisar0(SB),NOSPLIT,$0-8
+	// get Instruction Set Attributes 0 into x0
+	MRS	ID_AA64ISAR0_EL1, R0
+	MOVD	R0, ret+0(FP)
+	RET
+
+// func getisar1() uint64
+TEXT ·getisar1(SB),NOSPLIT,$0-8
+	// get Instruction Set Attributes 1 into x0
+	MRS	ID_AA64ISAR1_EL1, R0
+	MOVD	R0, ret+0(FP)
+	RET
+
+// func getpfr0() uint64
+TEXT ·getpfr0(SB),NOSPLIT,$0-8
+	// get Processor Feature Register 0 into x0
+	MRS	ID_AA64PFR0_EL1, R0
+	MOVD	R0, ret+0(FP)
+	RET
+
+// func getzfr0() uint64
+TEXT ·getzfr0(SB),NOSPLIT,$0-8
+	// get SVE Feature Register 0 into x0
+	MRS	ID_AA64ZFR0_EL1, R0
+	MOVD	R0, ret+0(FP)
+	RET