stdenv: libcxxabi dropped standalone in favor of useLLVM

The libcxxabi expression had a `standalone` argument in LLVM 7 which
triggered passing of the `-DLLVM_ENABLE_LIBCXX=ON` flag. In LLVM 11 this
flag and others are toggled by the `useLLVM` attribute of
`stdenv.hostPlatform` but this toggles a flag we don't need on Darwin.

Similar to the refactoring of LLVM 7 libc++abi by Ericson2314 in
3af7e984705, I have reintroduced the standalone argument for use in
bootstrapping the Darwin stdenv.

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

···
 { lib, stdenv, llvm_meta, cmake, fetch, libcxx, libunwind, llvm, version
+, standalone ? stdenv.hostPlatform.useLLVM or false
+, withLibunwind ? !stdenv.isDarwin && !stdenv.isFreeBSD && !stdenv.hostPlatform.isWasm
 , enableShared ? !stdenv.hostPlatform.isStatic
 }:
 
···
   ];
 
   nativeBuildInputs = [ cmake ];
-  buildInputs = lib.optional (!stdenv.isDarwin && !stdenv.isFreeBSD && !stdenv.hostPlatform.isWasm) libunwind;
+  buildInputs = lib.optional withLibunwind libunwind;
 
   cmakeFlags = lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"