[libomptarget][DeviceRTL] Generalise and simplify cmakelists

Step towards building the DeviceRTL for amdgpu.

Mostly replaces cuda-specific toolchain finding logic with the
generic logic currently found in the amdgpu deviceRTL cmake. Also
deletes dead code and changes the default to build on systems
without cuda installed, as the library doesn't use cuda and the
amdgpu-only systems generally won't have cuda installed.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D111983