C23 support for libm

Greetings!

I have started a project page [1] to track our effort to support C23
on FreeBSD.  Many new and exciting libm functions were added in this
standard revision, and it seems like so far we only have
{sin,cos,tan}pi{,f,l}() implemented (thanks to kargl for the work!)

If anybody is interested in working on this, please let me know.
I am not an expert in numerical programming, but will try ot look
into adding some of these if nobody else comes forth.

Yours,
Robert Clausecker

[1]: https://wiki.freebsd.org/C23

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