[PATCH] These builtin functions set errno. Mark them accordingly.

[Richard Smith]

>>! In D3806#12, @hfinkel wrote:
> The problem, as pointed out in the bug report, is that libstdc++ implements std::X in terms of __builtin_X

This is only non-conforming because `math_errhandling` is defined to a value that includes `MATH_ERRNO`. Another possible approach would be to subvert that macro instead of providing an `errno` guarantee in our builtins. Is there some reason we want to provide a guarantee that we set `errno` on domain error if the underlying libc does?

http://reviews.llvm.org/D3806