[Libomptarget] Update handling of architectures for DeviceRTL

The support for enabling and disabling certain architectures for the
OpenMP device RTL is different between AMD and Nvidia. This patch
updates the logic to make it common. This supports the `auto` format
more generally via the `nvptx-arch` and `amdgpu-arch` options. (These
are not availible at CMake time without a runtimes build, or another
install somewhere. But that only prevents users from using auto).

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D145513