[cmake] Add NATIVE build for cross compiling standalone builds
[cmake] Add NATIVE build for cross compiling standalone builds

TableGen is a host tool and requires a native variant for every build.
While building as a part of llvm this is trivial and llvm handles it.
However, building standalone means that lldb has to handle this itself.
Add a NATIVE build variant to enable this.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@366392 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb