fix: nix-shell environment for building spdk on arm

[maxwnewcomer]

This PR documents the changes needed to get the ARM64 build working and some of the issues encountered along the way.

1. Debugging the ARM Build

   • I spun up an arm64 machine on Hetzner and debugged the install process step by step.
   • First tried running nix-shell without SPDK and hit issues with pyelf. Replacing python3.withPackages with pkgs.python3Packages.pyelftools fixed that.

2. Key Issue

   • The main problem in a manual build was the -msse4 flag, which is for x86 extended instruction sets and doesn’t work on ARM.
   • I split the nix-shell compiler flags into configurable strings so we can set optimization flags per architecture.
   • Then, in the default nix-shell spdk configure and build, Clang was the default compiler, this led to SVE compilation issues. In the "no-spdk-installed" nix environment, I noticed I was getting success w/ GCC, so migrated the nix script to remove Clang and use GCC by default. I don't want to get presumptuous that this is the best solution, very open to feedback on this.

3. Context

   • As mentioned in the parent OEP ([ OEP 3817 ] - Support arm64 Deployments openebs#3817), the goal here is just to make ARM builds possible—not optimized.

Addresses

• Fixes Will not build on ARM #71

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Please let me know if you'd rather me focus on getting the OEP to implementable rather than starting to implement fixes!