[CMake] Rudimentary support for standalone CompilerRT build system.

This change allows to build compiler-rt libraries separately from
LLVM/Clang (path to LLVM build directory should be specified at
configure time). Running tests is not yet supported.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201647 91177308-0d34-0410-b5e6-96231b3b80d8