[CMake] Adding experimental support for tvOS and watchOS

Summary:
* Refactored the iOS config-ix.cmake code to be a more compact loop over supported embedded platforms.
* Added watchOS and tvOS as experimental platforms, they don't currently build so they are disabled by default

Reviewers: zaks.anna, kubabrecka, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16119

llvm-svn: 257544