llvm-build: Add --native-target and --enable-targets options, and add logic to

handle defining the "magic" target related components (like native,
nativecodegen, and engine).
 - We still require these components to be in the project (currently in
   lib/Target) so that we have a place to document them and hopefully make it
   more obvious that they are "magic".

llvm-svn: 144253