[libc][math][c++23] Add fmodbf16 math function (#155575)

This PR adds fmodbf16 basic math function for BFloat16 type along with
the tests.

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Signed-off-by: Krishna Pandey <kpandey81930@gmail.com>
GitOrigin-RevId: 96b1dfb57fc822ea69bf75a8f0a69eb168776faa
22 files changed