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/10/default.nix

@@ -185,7 +185,7 @@ let
 
     openmp = callPackage ./openmp.nix {};
 
-    libunwind = callPackage ./libunwind.nix ({} //
+    libunwind = callPackage ./libunwind ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
       }));

pkgs/development/compilers/llvm/11/default.nix

@@ -187,7 +187,7 @@ let
 
     openmp = callPackage ./openmp.nix {};
 
-    libunwind = callPackage ./libunwind.nix ({} //
+    libunwind = callPackage ./libunwind ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
       }));

pkgs/development/compilers/llvm/8/default.nix

@@ -185,7 +185,7 @@ let
 
     openmp = callPackage ./openmp.nix {};
 
-    libunwind = callPackage ./libunwind.nix ({} //
+    libunwind = callPackage ./libunwind ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
       }));

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

@@ -185,7 +185,7 @@ let
 
     openmp = callPackage ./openmp.nix {};
 
-    libunwind = callPackage ./libunwind.nix ({} //
+    libunwind = callPackage ./libunwind ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
       }));