llvm_12: fix cross-compilation

Cross-compilation is broken because the method of finding ncurses has
changed, causing the build for the 'build system' to fail with a linking
error due to ncurses being for the 'host system' (where you're compiling
for).

This patch disables ncurses, which is not a very neat solution, but will
do until someone takes this upstream and gets it fixed properly.

Closes https://github.com/NixOS/nixpkgs/issues/127946.

Error that's seen before applying this:

/nix/store/hash-binutils-2.35.1/bin/ld: /nix/store/hash-ncurses-6.2-aarch64-unknown-linux-gnu/lib/libtinfo.so: error adding symbols: file in wrong format

pkgs/development/compilers/llvm/12/llvm/default.nix

@@ -54,7 +54,8 @@ in stdenv.mkDerivation (rec {
   buildInputs = [ libxml2 libffi ]
     ++ optional enablePFM libpfm; # exegesis
 
-  propagatedBuildInputs = [ ncurses zlib ];
+  propagatedBuildInputs = optionals (stdenv.buildPlatform == stdenv.hostPlatform) [ ncurses ]
+    ++ [ zlib ];
 
   patches = [
     ./gnu-install-dirs.patch