llvmPackages_13.libcxxabi: Align with 11 libcxxabi

LLVM 11 libcxxabi has some flags to support usage in the Darwin stdenv,
in particular, `standalone` and `withLibunwind`.

Darwin stdenv needs the `standalone` flag because its `hostPlatform` set
doesn't have `useLLVM` set to true. And it needs `withLibunwind` to
explicitly disable including `libunwind` as a build input.

We also prefix `install_name_tool` in case we're cross-compiling.

pkgs/development/compilers/llvm/13/libcxxabi/default.nix

@@ -1,6 +1,7 @@
-{ lib, stdenv, llvm_meta, cmake, python3, src, libunwind, version
+{ lib, stdenv, llvm_meta, cmake, python3, src, libunwind, libcxx, version
 , enableShared ? !stdenv.hostPlatform.isStatic
-, libcxx
+, standalone ? stdenv.hostPlatform.useLLVM or false
+, withLibunwind ? !stdenv.isDarwin && !stdenv.isFreeBSD && !stdenv.hostPlatform.isWasm
 }:
 
 stdenv.mkDerivation rec {
@@ -23,12 +24,13 @@ stdenv.mkDerivation rec {
   ];
 
   nativeBuildInputs = [ cmake python3 ];
-  buildInputs = lib.optional (!stdenv.isDarwin && !stdenv.isFreeBSD && !stdenv.hostPlatform.isWasm) libunwind;
+  buildInputs = lib.optional withLibunwind libunwind;
 
   cmakeFlags = [
     "-DLIBCXXABI_LIBCXX_INCLUDES=${libcxx.dev}/include/c++/v1"
-  ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
+  ] ++ lib.optionals standalone [
     "-DLLVM_ENABLE_LIBCXX=ON"
+  ] ++ lib.optionals (standalone && withLibunwind) [
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"
   ] ++ lib.optionals stdenv.hostPlatform.isWasm [
     "-DLIBCXXABI_ENABLE_THREADS=OFF"