[CMake] Add experimental support for building compiler-rt for iOS

Summary:
This is a reunification of three separate reviews D11073, D11082, D11083.

Having them separate was not constructive even though the patches were smaller because it led to fragmented conversations, and this is really all about one change.

This patch incorporates feedback from samsonov, and refactors the hacky darwin code out of the root CMakeLists.txt and int config-ix.cmake.

Reviewers: zaks.anna, bogner, kubabrecka, chandlerc, samsonov

Subscribers: jevinskie, filcab, llvm-commits

Differential Revision: http://reviews.llvm.org/D11820

llvm-svn: 244948