llvmPackages_*.libunwind: Move into own directory

I expect to soon give it some patches that would go in there too, but
either way it's good to match the others.

pkgs/development/compilers/llvm/9/libunwind/default.nix

@@ -0,0 +1,14 @@
+{ lib, stdenv, version, fetch, cmake, fetchpatch
+, enableShared ? !stdenv.hostPlatform.isStatic
+}:
+
+stdenv.mkDerivation rec {
+  pname = "libunwind";
+  inherit version;
+
+  src = fetch pname "1wb02ha3gl6p0a321hwpll74pz5qvjr11xmjqx62g288f1m10njk";
+
+  nativeBuildInputs = [ cmake ];
+
+  cmakeFlags = lib.optional (!enableShared) "-DLIBUNWIND_ENABLE_SHARED=OFF";
+}