[libc][math] Refactor fmaximum_mag_num family to header-only (#182169)

Refactors the fmaximum_mag_num math family to be header-only.

Closes https://github.com/llvm/llvm-project/issues/182168

Target Functions:
  - fmaximum_mag_num
  - fmaximum_mag_numbf16
  - fmaximum_mag_numf

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Co-authored-by: Muhammad Bassiouni <60100307+bassiounix@users.noreply.github.com>
GitOrigin-RevId: 23cc957f50710b431854e5281a434fcdc419c284