Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

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

vendor/modernc.org/libc/tls_linux_amd64.s

@@ -0,0 +1,40 @@
+#include "funcdata.h"
+#include "textflag.h"
+
+TEXT ·TLSAlloc(SB),$24-24
+	GO_ARGS
+	NO_LOCAL_POINTERS
+	MOVQ p0+0(FP), AX
+	MOVQ AX, 0(SP)
+	MOVQ p1+8(FP), AX
+	MOVQ AX, 8(SP)
+	CALL ·tlsAlloc(SB)
+	MOVQ 16(SP), AX
+	MOVQ AX, ret+16(FP)
+	RET
+
+TEXT ·TLSFree(SB),$16-16
+	GO_ARGS
+	NO_LOCAL_POINTERS
+	MOVQ p0+0(FP), AX
+	MOVQ AX, 0(SP)
+	MOVQ p1+8(FP), AX
+	MOVQ AX, 8(SP)
+	CALL ·tlsFree(SB)
+	RET
+
+TEXT ·TLSAllocaEntry(SB),$8-8
+	GO_ARGS
+	NO_LOCAL_POINTERS
+	MOVQ p0+0(FP), AX
+	MOVQ AX, 0(SP)
+	CALL ·tlsAllocaEntry(SB)
+	RET
+
+TEXT ·TLSAllocaExit(SB),$8-8
+	GO_ARGS
+	NO_LOCAL_POINTERS
+	MOVQ p0+0(FP), AX
+	MOVQ AX, 0(SP)
+	CALL ·tlsAllocaExit(SB)
+	RET