[PATCH] test-assert-c++-variadic.cc: Disable assert_works for GCC 14.2 or later

[Florian Weimer]

* Tomasz Kaminski:

> Could you elaborate? clang defines __clang__ to value 1.
> And if the __clang__ macro is not present, || __clang__ is false.

We build glibc with -Werror=undef, so this results in a compilation
error with gcc-toolset-14-gcc-14.2.1-12.el9_7.x86_64 (for example).

Thanks,
Florian