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

[Jon Roelofs via llvm-commits]

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@@ -1,4 +1,4 @@
-//===-- cpu_model.c - Support for __cpu_model builtin  ------------*- C -*-===//
+//===-- cpu_model_x86.c - Support for __cpu_model builtin  --------*- C -*-===//
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jroelofs wrote:

- [ ] FIXME: rename file in title comment.

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