[libc][math][c++23] Add fmodbf16 math function (#155575) This PR adds fmodbf16 basic math function for BFloat16 type along with the tests. --------- Signed-off-by: Krishna Pandey <kpandey81930@gmail.com> GitOrigin-RevId: 96b1dfb57fc822ea69bf75a8f0a69eb168776faa