Change the CMake build to generate full debugging info when COMPILER_RT_DEBUG=ON

The compiler-rt CMake build currently uses -gline-tables-only for all builds. While this makes the debugger correctly map source files, debugging of the runtime libraries in general is pretty tough, because you don't see any local variables, parameters, etc. Let's allow emitting full debugging info by setting COMPILER_RT_DEBUG=ON.

Reviewed at http://reviews.llvm.org/D6764