Add two more special values for LIBOMPTARGET_DEVICE_ARCHITECTURES: `amdgpu` and `nvptx`, to support building for all AMDGPU and NVPTX targets respectively. This can be used in place of `all` when offload is built with one of the GPU plugins only.
Add two more special values for LIBOMPTARGET_DEVICE_ARCHITECTURES: `amdgpu` and `nvptx`, to support building for all AMDGPU and NVPTX targets respectively. This can be used in place of `all` when offload is built with one of the GPU plugins only.