[libc++][math] Mathematical special functions: Implementing std::legendre

[PaulXiCao]

Implementing the Legendre polynomials std::legendre, std::legendref, std::legendrel which are part of C++17's mathematical special functions.

I started out from this abandoned merge request: https://reviews.llvm.org/D58876.

Documentation in libcxx/docs/Status/ is not yet included. This will be added after the integration of #89982.

As the polynomials are normalized one does not need to fallback to computations using double precision for float input (opposed to other special functions).

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