[CMake] Support builtins as Clang default rtlib in compiler-rt

Use compiler-rt builtins when selected as default Clang rtlib and avoid
explicitly passing -rtlib= flag to avoid the "argument unused during
compilation" warning.

This is a partial alternative to D47094 that does not rely on compiler
runtime checks.

Differential Revision: https://reviews.llvm.org/D47115

llvm-svn: 333037