1) Adding support to invoke CLang/Linker for Mac

2) Expecting AppDep native libs to be under <AppDepRoot>\CPPSdk\<OS>\x64 folder instead of <AppDepRoot>
Update the references to Microsoft.DotNet.ILCompiler and Microsoft.DotNet.AppDep packages with Mac CPPCodegen support.

scripts/build/build_appdeps.sh

@@ -7,11 +7,6 @@ SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 
 source $SCRIPT_DIR/../_common.sh
 
-# Dotnet-compile-native doesn't work for mac yet
-if [[ "$(uname)" != "Linux" ]]; then
-	exit 0
-fi
-
 REPO_ROOT="$SCRIPT_DIR/../.."
 APPDEPS_PROJECT_DIR=$REPO_ROOT/src/Microsoft.DotNet.Tools.Compiler.Native/appdep