[CMake] Fix compiler-rt builtins configuration bug on darwin

[dutkalex]

This PR fixes a configuration bug (#173810) that was particularly painful to track down because it happens in the compiler-rt builtins configuration, which is performed on the fly in the middle of the build step.

The symptoms
On recent macos systems, the old i386 arch is disabled even if the system SDK lists it as supported. However, while all the other subprojects correctly diagnose the situation and adequately remove i386 from the list of architectures to compile for, the builtins behave differently and fails to build.

The root cause
When configuring the builtins the variable TEST_COMPILE_ONLY is ON (see. https://github.com/llvm/llvm-project/blob/main/compiler-rt/cmake/builtin-config-ix.cmake#L219), and because the cmake code performing this logic is located in an if(NOT TEST_COMPILE_ONLY) block, the code is not executed in this specific context.

The proposed solution
Move this piece of logic outside of the conditional block so that all code paths are treated equally.

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