[Bug libstdc++/100444] std::random_device isn't random on AMD

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100444

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
So at least libstdc++ does check RDRAND availability via cpuid.  It also
already
checks the return value for "no avaialble randomness":

      while (__builtin_ia32_rdrand32_step(&val) == 0)
        if (--retries == 0)
          std::__throw_runtime_error(__N("random_device: rdrand failed"));

so here it could check for -1 as well though in theory that
can happen with true randomness as well, even if very unlikely.  Note
that it would never return -1 then (as it never returns 0 at the moment).

Thus I believe the issue is mitigated at the kernel level and people that
cannot be bothered to update their ucode or the kernel are not likely
bothered to update libstdc++ either.  Note handling it above would
raise a runtime error rather than falling back to other sources of randomness.
Doing the checking where we detect rdrand support would avoid that.