ghc: use CXX=c++, not CXX=cxx

Otherwise attempt to build ghcHEAD from local checkout fails as:

$ nix build -L --impure --expr 'with import ~/nm {}; haskell.compiler.ghcHEAD.overrideAttrs (oa: { src = ./.; patches = []; nativeBuildInputs = oa.nativeBuildInputs ++ [ git ]; })' --keep-failed
...
ghc> checking C++ standard library flavour... ./configure: line 11487: /nix/store/r7r10qvsqlnvbzjkjinvscjlahqbxifl-gcc-wrapper-11.3.0/bin/cxx: No such file or directory

I think 'cxx' is not provided by stdenv.

pkgs/development/compilers/ghc/8.10.7.nix

···
     # GHC is a bit confused on its cross terminology, as these would normally be
     # the *host* tools.
     export CC="${targetCC}/bin/${targetCC.targetPrefix}cc"
-    export CXX="${targetCC}/bin/${targetCC.targetPrefix}cxx"
+    export CXX="${targetCC}/bin/${targetCC.targetPrefix}c++"
     # Use gold to work around https://sourceware.org/bugzilla/show_bug.cgi?id=16177
     export LD="${targetCC.bintools}/bin/${targetCC.bintools.targetPrefix}ld${lib.optionalString useLdGold ".gold"}"
     export AS="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}as"

pkgs/development/compilers/ghc/8.8.4.nix

···
     # GHC is a bit confused on its cross terminology, as these would normally be
     # the *host* tools.
     export CC="${targetCC}/bin/${targetCC.targetPrefix}cc"
-    export CXX="${targetCC}/bin/${targetCC.targetPrefix}cxx"
+    export CXX="${targetCC}/bin/${targetCC.targetPrefix}c++"
     # Use gold to work around https://sourceware.org/bugzilla/show_bug.cgi?id=16177
     export LD="${targetCC.bintools}/bin/${targetCC.bintools.targetPrefix}ld${lib.optionalString useLdGold ".gold"}"
     export AS="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}as"

pkgs/development/compilers/ghc/9.0.2.nix

···
     # GHC is a bit confused on its cross terminology, as these would normally be
     # the *host* tools.
     export CC="${targetCC}/bin/${targetCC.targetPrefix}cc"
-    export CXX="${targetCC}/bin/${targetCC.targetPrefix}cxx"
+    export CXX="${targetCC}/bin/${targetCC.targetPrefix}c++"
     # Use gold to work around https://sourceware.org/bugzilla/show_bug.cgi?id=16177
     export LD="${targetCC.bintools}/bin/${targetCC.bintools.targetPrefix}ld${lib.optionalString useLdGold ".gold"}"
     export AS="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}as"

pkgs/development/compilers/ghc/9.2.3.nix

···
     # GHC is a bit confused on its cross terminology, as these would normally be
     # the *host* tools.
     export CC="${targetCC}/bin/${targetCC.targetPrefix}cc"
-    export CXX="${targetCC}/bin/${targetCC.targetPrefix}cxx"
+    export CXX="${targetCC}/bin/${targetCC.targetPrefix}c++"
     # Use gold to work around https://sourceware.org/bugzilla/show_bug.cgi?id=16177
     export LD="${targetCC.bintools}/bin/${targetCC.bintools.targetPrefix}ld${lib.optionalString useLdGold ".gold"}"
     export AS="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}as"

pkgs/development/compilers/ghc/head.nix

···
     # GHC is a bit confused on its cross terminology, as these would normally be
     # the *host* tools.
     export CC="${targetCC}/bin/${targetCC.targetPrefix}cc"
-    export CXX="${targetCC}/bin/${targetCC.targetPrefix}cxx"
+    export CXX="${targetCC}/bin/${targetCC.targetPrefix}c++"
     # Use gold to work around https://sourceware.org/bugzilla/show_bug.cgi?id=16177
     # and more generally have a faster linker.
     export LD="${targetCC.bintools}/bin/${targetCC.bintools.targetPrefix}ld${lib.optionalString useLdGold ".gold"}"