[flang][runtime] Enable more code for offload device builds. (#67489)

I extended the "closure" of the device code containing the initial
transformational.cpp. The device side of the library should not be
complete at least for some APIs. For example, I tested with C OpenMP
code calling BesselJnX0 with a nullptr descriptor that failed with
a runtime error when executing on a GPU.

I added `--expt-relaxed-constexpr` for NVCC compiler to avoid multiple
warnings about missing `__attribute__((device))` on constexpr methods
coming from C++ header files.