[libc++] Move math special functions under __cmath (#213944)

It was always a bit weird that these were the only functions under
__math/ which were not in the __math namespace and were not in the
global namespace. This also led to a workaround in the C++20 modules
testing.

Instead, move math special functions under __cmath/, which will contain
APIs that are part of `<cmath>` but not `<math.h>`.

GitOrigin-RevId: 8d47bda9cd1ab5ab27295ed15eaded4084c6201c