[openmp][devicertl] Freestanding nvptx via stub printf

Compiled nvptx devicertl as freestanding, breaking the
dependency on host glibc and gcc-multilibs. Thus build it by default.

Comes at the cost of #defining out printf. Tried mapping it onto
__builtin_printf but that gets transformed back to printf instead
of hitting the cuda/openmp lowering transform.

Printf could be preserved by one of:
- dropping all the standard headers and ffreestanding
- providing a header only printf implementation
- changing the compiler handling of printf

Reviewed By: grokos

Differential Revision: https://reviews.llvm.org/D108349