[Issue 18247] core.stdc.math functions that never set errno should be pure

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18247

--- Comment #1 from Nathan S. <n8sh.secondary at hotmail.com> ---
cos, sin, tan should be removed from list of pure functions. On some platforms
they set errno, on others not. See discussion at
https://github.com/dlang/druntime/pull/2045

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