math::nolibc: atanh

[MendocinoWhiteDeer]

Added atanh nolibc definitions (not to be confused with atan!) and more atanh test points in the math_tests module. atanh relies on log1p, which was also defined. These definitions were translated from FreeBSD (see here) and musl (see here).

If anybody would like to verify that these nolibc functions are correct, you can do so in a libc environment with the feature C3_MATH. This replaces libc definitions with nolibc definitions when they exist and defaults to libc when they don't. If anybody wants to test the authenticity of these functions run

c3c compile-test -D C3_MATH test/unit/stdlib/math/math.c3

[lerno]

Thank you!