[libc][math][c23] Add cospif16 function (#113001)

Implementation of `cos` for half precision floating point inputs scaled
by pi (i.e., `cospi`), correctly rounded for all rounding modes.

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Co-authored-by: OverMighty <its.overmighty@gmail.com>
GitOrigin-RevId: 7395ef5419a6438f0c48685bf00b7f151178743d
15 files changed