llvmPackages.clang: use the system libunwind on Darwin

While the Darwin stdenv bootstrap sets up its own clang wrappers and
doesn’t provide these wrappers in the final stdenv, it does use them
indirectly via the LLVM bootstrap to build LLVM and its libraries.

Note on using the system libunwind: It is possible to build and use the
LLVM libunwind on Darwin, but using the system by default one ensures
everything is using the same unwinder.

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

@@ -781,16 +781,20 @@ let
           bintools = bintools';
           extraPackages =
             [ targetLlvmLibraries.compiler-rt-no-libc ]
-            ++ lib.optionals (!stdenv.targetPlatform.isWasm && !stdenv.targetPlatform.isFreeBSD) [
-              targetLlvmLibraries.libunwind
-            ];
+            ++ lib.optionals
+              (
+                !stdenv.targetPlatform.isWasm && !stdenv.targetPlatform.isFreeBSD && !stdenv.targetPlatform.isDarwin
+              )
+              [
+                targetLlvmLibraries.libunwind
+              ];
           extraBuildCommands =
             lib.optionalString (lib.versions.major metadata.release_version == "13") (
               ''
                 echo "-rtlib=compiler-rt -Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
                 echo "-B${targetLlvmLibraries.compiler-rt-no-libc}/lib" >> $out/nix-support/cc-cflags
               ''
-              + lib.optionalString (!stdenv.targetPlatform.isWasm) ''
+              + lib.optionalString (!stdenv.targetPlatform.isWasm && !stdenv.targetPlatform.isDarwin) ''
                 echo "--unwindlib=libunwind" >> $out/nix-support/cc-cflags
                 echo "-L${targetLlvmLibraries.libunwind}/lib" >> $out/nix-support/cc-ldflags
               ''
@@ -811,7 +815,7 @@ let
               "-B${targetLlvmLibraries.compiler-rt-no-libc}/lib"
             ]
             ++ lib.optional (
-              !stdenv.targetPlatform.isWasm && !stdenv.targetPlatform.isFreeBSD
+              !stdenv.targetPlatform.isWasm && !stdenv.targetPlatform.isFreeBSD && !stdenv.targetPlatform.isDarwin
             ) "--unwindlib=libunwind"
             ++ lib.optional (
               !stdenv.targetPlatform.isWasm
@@ -820,7 +824,7 @@ let
             ) "-lunwind"
             ++ lib.optional stdenv.targetPlatform.isWasm "-fno-exceptions";
           nixSupport.cc-ldflags = lib.optionals (
-            !stdenv.targetPlatform.isWasm && !stdenv.targetPlatform.isFreeBSD
+            !stdenv.targetPlatform.isWasm && !stdenv.targetPlatform.isFreeBSD && !stdenv.targetPlatform.isDarwin
           ) [ "-L${targetLlvmLibraries.libunwind}/lib" ];
         }
       );