[Libomptarget] Build the DeviceRTL BC using clang directly

Currently the bitcode library is build using the clang front-end
manually. This was originally done because we did not support device
only compilation. Now we support device only compilation, at least for a
single offloading toolchain, so we can instead use clang directly rather
than using the front-end. This saves us needing to define things like
`aux_triple`.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D130293