[libomptarget][nfc]Generalise DeviceRTL cmake to allow building for amdgpu

Essentially moves the foreach over sm integers into a macro and instantiates it for nvptx.

NFC in that the macro is not presently instantiated for amdgpu as the corresponding code doesn't compile yet.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D111987