[CMake] Update lit test configuration

There are several changes:
- Don't stringify Pythonized bools (that's why we're Pythonizing them)
- Support specifying target and sysroot via CMake variables
- Use consistent spelling for --target, --sysroot, --gcc-toolchain

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@353137 91177308-0d34-0410-b5e6-96231b3b80d8