Update libm tests from NetBSD

[arichardson]

Summary

I did this without a full vendor update since that would cause too many
conflicts. Since these files now almost match the NetBSD sources the
next git subtree merge should work just fine.

[lwhsu]

Those tests currently pass, right?

[arichardson]

In D28797#645793, @lwhsu wrote:

Those tests currently pass, right?

Yes, they do, but

contrib/netbsd-tests/lib/libm/t_cbrt.c

313

Yes, they pass after re-applying the (long double)1.0 / 3.0 patch. It works on NetBSD because powl is just an alias for pow there (i.e. they only have double versions not long double ones).