[compiler-rt] [builtins] Refactor cpu_model support to reduce #if nesting. NFCI (PR #75635)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo commented:

I think this looks reasonable overall, but there's a typo in the name of Fuchsia, which I believe would lead to its codepaths being entirely missed :-)

Thanks, in this form I think the ifdef jungle is much more manageable.

https://github.com/llvm/llvm-project/pull/75635