Universal CPU Build Support for Qiskit Aer with CUDA and ROCm

[basnijholt]

This PR introduces an update to the qiskit_aer build process, ensuring that the CPU variant of controller_wrappers is always built, alongside CUDA and ROCm variants. This ensures that builds targeting CUDA or ROCm can still operate on systems that only have CPU capabilities.

Key Changes:

1. CPU Build as a Default: The CPU backend (controller_wrappers_cpu) is now always built. This is independent of whether CUDA (controller_wrappers_cuda) or ROCm (controller_wrappers_rocm) backends are being built.

2. Conditional CUDA and ROCm Builds: Compilation of CUDA and ROCm backends remains conditional based on the AER_THRUST_BACKEND setting. This ensures that these backends are built only when specified, without affecting the CPU build.

3. Organized Build Process: The build process in CMakeLists.txt is organized to handle CPU, CUDA, and ROCm backends separately. Each backend has its specific compiler flags, definitions, and dependencies, ensuring a clean and manageable build process.

This update significantly improves the flexibility and applicability of Qiskit Aer, enabling it to run on a broader range of systems, from specialized CUDA and ROCm environments to general-purpose CPU-only machines.

I my case this is useful because we distribute a single environment / container that run both on CPU and GPU machines. ATM qiskit-aer is our only dependency that doesn't allow this mode of operation.

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[basnijholt]

One could also add an environment variable to for example force the CPU implementation.

But that can be done in a followup PR.

[basnijholt]

@doichanj, thanks a lot for looking at this PR!

This problem is still very annoying for us because we're maintaining 2 locked environments for CUDA and non-CUDA now. It would be awesome to be able to use one installation that works both on CPU and CUDA.

[doichanj]

I could not build with this PR when setting -DAER_THRUST_BACKEND=CUDA, it seems CPU build is not correctly configured and failed