[Libomptarget] Build the device library even if the sm list is empty

We previously had some logic that stopped us from building the device runtime if
there were no NVPTX architectures provided. This is incorrect because we could
have AMDGPU libraries. Even if the lists are empty we should be able to attempt
to build these and get dummy output. THis wilil make it much easier for our
tooling which expects certain libraries. If the user wishes to disable the
library entirely they should use `-DLIBOMPTARGET_BUILD_DEVICERTL_BCLIB=OFF"

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D130266