[OpenMP][NVPTX] Disable building NVPTX deviceRTL by default on a non-CUDA system

D95466 dropped CUDA to build NVPTX deviceRTL and enabled it by default.
However, the building requires some libraries that are not available on non-CUDA
system by default, which could break the compilation. This patch disabled the
build by default. It can be enabled with `LIBOMPTARGET_BUILD_NVPTX_BCLIB=ON`.

Reviewed By: kparzysz

Differential Revision: https://reviews.llvm.org/D95556