Build alpaka code in GPU-only mode

Build alpaka device code for the CUDA and ROCm back-ends in "GPU only" mode.
In this mode, functions marked as ALPAKA_FN_ACC are marked as __device__
functions, and are compiled only for the corresponding GPU device back-ends.

Currently, functions marked as ALPAKA_FN_ACC are marked as __host__ __device__
functions, and may be compiled for both device and host back-ends. The latter
leads to linker errors in kernels that use device symbols like threadIdx,
blockIdx, etc. that are compiled for the ROCm back-end.

scram-tools.file/tools/alpaka/alpaka-cuda.xml

@@ -2,8 +2,8 @@
   <use name="alpaka"/>
   <use name="cuda"/>
   <!-- host comiplation should run with ALPAKA_HOST_ONLY defined -->
-  <flags CXXFLAGS="-DALPAKA_ACC_GPU_CUDA_ENABLED -DALPAKA_HOST_ONLY"/>
-  <flags GENREFLEX_CPPFLAGS="-DALPAKA_ACC_GPU_CUDA_ENABLED -DALPAKA_HOST_ONLY"/>
+  <flags CXXFLAGS="-DALPAKA_ACC_GPU_CUDA_ENABLED -DALPAKA_ACC_GPU_CUDA_ONLY_MODE -DALPAKA_HOST_ONLY"/>
+  <flags GENREFLEX_CPPFLAGS="-DALPAKA_ACC_GPU_CUDA_ENABLED -DALPAKA_ACC_GPU_CUDA_ONLY_MODE -DALPAKA_HOST_ONLY"/>
   <!-- device comiplation should run without ALPAKA_HOST_ONLY defined -->
-  <flags CUDA_FLAGS="-DALPAKA_ACC_GPU_CUDA_ENABLED -UALPAKA_HOST_ONLY"/>
+  <flags CUDA_FLAGS="-DALPAKA_ACC_GPU_CUDA_ENABLED -DALPAKA_ACC_GPU_CUDA_ONLY_MODE -UALPAKA_HOST_ONLY"/>
 </tool>

scram-tools.file/tools/alpaka/alpaka-rocm.xml

@@ -2,8 +2,8 @@
   <use name="alpaka"/>
   <use name="rocm"/>
   <!-- host comiplation should run with ALPAKA_HOST_ONLY defined -->
-  <flags CXXFLAGS="-DALPAKA_ACC_GPU_HIP_ENABLED -DALPAKA_HOST_ONLY"/>
-  <flags GENREFLEX_CPPFLAGS="-DALPAKA_ACC_GPU_HIP_ENABLED -DALPAKA_HOST_ONLY"/>
+  <flags CXXFLAGS="-DALPAKA_ACC_GPU_HIP_ENABLED -DALPAKA_ACC_GPU_HIP_ONLY_MODE -DALPAKA_HOST_ONLY"/>
+  <flags GENREFLEX_CPPFLAGS="-DALPAKA_ACC_GPU_HIP_ENABLED -DALPAKA_ACC_GPU_HIP_ONLY_MODE -DALPAKA_HOST_ONLY"/>
   <!-- device comiplation should run without ALPAKA_HOST_ONLY defined -->
-  <flags ROCM_FLAGS="-DALPAKA_ACC_GPU_HIP_ENABLED -UALPAKA_HOST_ONLY"/>
+  <flags ROCM_FLAGS="-DALPAKA_ACC_GPU_HIP_ENABLED -DALPAKA_ACC_GPU_HIP_ONLY_MODE -UALPAKA_HOST_ONLY"/>
 </tool>