llvm: support cross compilation with useLLVM flag

You can build (partially) with LLVM toolchain using the useLLVM flag.
This works like so:

  nix-build -A hello --arg crossSystem '{ system =
    "aarch64-unknown-linux-musl"; useLLVM = true }'

also don’t separate debug info in lldClang

It doesn’t work currently with that setup hook. Missing build-id?

pkgs/test/cross/default.nix

@@ -26,14 +26,20 @@ let
     # We need to remove whitespace, unfortunately
     # Windows programs use \r but Unix programs use \n
 
+    echo Running native-built program natively
+
     # find expected value natively
     ${getExecutable hostPkgs pkgFun exec} ${args'} \
       | dos2unix > $out/expected
 
+    echo Running cross-built program in emulator
+
     # run emulator to get actual value
     ${emulator} ${getExecutable crossPkgs pkgFun exec} ${args'} \
       | dos2unix > $out/actual
 
+    echo Comparing results...
+
     if [ "$(cat $out/actual)" != "$(cat $out/expected)" ]; then
       echo "${pkgName} did not output expected value:"
       cat $out/expected