core.stdc.[tg]math: Revise for FreeBSD

[kinke]

Based on info spit out by https://www.freebsd.org/cgi/man.cgi?sektion=3.

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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dub run digger -- build "master + druntime#3119"