[Bug libstdc++/80506] Wrong magic number in std::gamma_distribution

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80506

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is fixed on trunk, I'm wondering whether to backport it to gcc-6-branch
and gcc-7-branch.