[alpakatest] Support all alpaka backends at the same time

[fwyzard]

Support serial, TBB, CUDA and ROCm at the same time.

Split compilation by backend:

• the ALPAKA_..._ENABLED macros are only defined one at a time;
• introduce new ALPAKA_..._PRESENT macros to identify all backends for which support is being compiled;
• link all libraries to the main executable, including the "portable", backend-specific ones.

Add forward declaration for alpaka templates and types (thanks to Matti for the idea). Add explicit instantiation definitions and declarations to the initialisation code, and move it to the AlpakaCore "portable" library. Use new pinned host memory functionality, introduced in the latest alpaka update.

Update alpaka the fwyzard/develop private branch, pending integration upstream. Relevant changes include:

• implement separate types for the CUDA and HIP/ROCm backends;
• add a new API to allocate pinned host memory (pending);
• add ALPAKA_DEFAULT_HOST_MEMORY_ALIGNMENT macro (pending).

[fwyzard]

Compiled with, for example

make -j`nproc` alpakatest CUDA_BASE=/usr/local/cuda-11.5.2 CUDA_ARCH=61 ROCM_BASE=/opt/rocm-5.0.2

the rsulting alpakatest binary supports --serial, --tbb, --cuda and --hip.

[fwyzard]

(I accidentally deleted this branch, I re-opened a new PR at #357)