[CUDA][NFC] Split math.h functions out of __clang_cuda_device_functions.h

This is not supported to change anything but allow us to reuse the math
functions separately from the device functions, e.g., source them at
different times. This will be used by the OpenMP overlay.

This also adds two `return` keywords that were missing.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D77238