[offload] Support LIBOMPTARGET_DEVICE_ARCHITECTURES={amdgpu|nvptx} (#119070)

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.

Author: mgorny

offload/DeviceRTL/CMakeLists.txt

@@ -57,6 +57,10 @@ set(LIBOMPTARGET_DEVICE_ARCHITECTURES "all" CACHE STRING
 
 if(LIBOMPTARGET_DEVICE_ARCHITECTURES STREQUAL "all")
   set(LIBOMPTARGET_DEVICE_ARCHITECTURES ${all_gpu_architectures})
+elseif(LIBOMPTARGET_DEVICE_ARCHITECTURES STREQUAL "amdgpu")
+  set(LIBOMPTARGET_DEVICE_ARCHITECTURES ${all_amdgpu_architectures})
+elseif(LIBOMPTARGET_DEVICE_ARCHITECTURES STREQUAL "nvptx")
+  set(LIBOMPTARGET_DEVICE_ARCHITECTURES ${all_nvptx_architectures})
 elseif(LIBOMPTARGET_DEVICE_ARCHITECTURES STREQUAL "auto" OR
        LIBOMPTARGET_DEVICE_ARCHITECTURES STREQUAL "native")
   if(NOT LIBOMPTARGET_NVPTX_ARCH AND NOT LIBOMPTARGET_AMDGPU_ARCH)