[clang] [CUDA] fix wrapper cmath header to match #136101 (PR #139164)

[Artem Belevich via cfe-commits]

Artem-B wrote:

@jhuber6 @ldionne One concern I have for this change is that it will break folks who will use older libc++ with the new Clang + wrapper headers.

Is older libc++ expected to work with non-matching clang version? If the expectation is that libc++ and clang are from the same version, then adapting the wrappers to match the current libc++ is sufficient, and there's no need to provide extra tweaks for backward compatibility.

I guess I can mitigate it to some degree by keeping the old variant of the change with `#if _LIBCPP_VERSION < 210000`, but I wonder if I need to do that at all.



https://github.com/llvm/llvm-project/pull/139164