[OpenMP][NFC] Rename OmptCallback.cpp into OpenMP/OMPT/Callback.cpp (#73813)

Also revert the ifdef OMPT_SUPPORT order to have the short fallback
first and not after 400 lines.

GitOrigin-RevId: bdecfebce4b0af94b3afa3e20add0b9d232f21ad
2 files changed