[CMake] Determine which compiler-rt libraries are supported on
a given platform in a top-level CMakeLists.txt to use it both
in lib/ and in test/ subdirectories. Move architecture/platform
checks to config-ix.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@215247 91177308-0d34-0410-b5e6-96231b3b80d8