[libcxx][NFC] Use macros for functions that support overriding detection (#133876)

We plan to replace the existing mechanism for overriding detection with
one that doesn't require the use of a special section as an alternative
to llvm/llvm-project#120805 which had other downsides.

This change is a pure refactoring that lays the foundation for a
subsequent change that will introduce the new detection mechanism.

GitOrigin-RevId: 25a03c1c7cec62ad7b6ed7a176df4e91ba968332
1 file changed