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

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".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144253 91177308-0d34-0410-b5e6-96231b3b80d8