fixup! [builtins] Refactor cpu_model support to reduce #if nesting. NFCI

https://github.com/llvm/llvm-project/pull/75635#issuecomment-1863433937

```
/b/s/w/ir/x/w/llvm_build/./bin/clang --target=aarch64-unknown-linux-gnu --sysroot=/b/s/w/ir/x/w/cipd/linux -DHAS_ASM_LSE -DVISIBILITY_HIDDEN  --target=aarch64-unknown-linux-gnu -O2 -g -DNDEBUG -fno-lto -std=c11 -fPIC -fno-builtin -fvisibility=hidden -fomit-frame-pointer -DCOMPILER_RT_HAS_FLOAT16 -MD -MT CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o -MF CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o.d -o CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o -c /b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/lib/builtins/cpu_model/aarch64.c
In file included from /b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/lib/builtins/cpu_model/aarch64.c:43:
/b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/lib/builtins/cpu_model/aarch64/lse_atomics/sysauxv.inc:5:41: error: use of undeclared identifier 'HWCAP_ATOMICS'
    5 |   __aarch64_have_lse_atomics = (hwcap & HWCAP_ATOMICS) != 0;
      |                                         ^
1 error generated.
```