[Bug c/71255] Implement #pragma may_alias

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71255

--- Comment #28 from Florian Weimer <fw at gcc dot gnu.org> ---
We can put such a version check into the glibc headers and see how it works out
in practice.  As long as there is consensus to fix any related breakage
(related to the attribute and forward declarations) for the GCC 7 release, this
should work.  I think we won't need a may_alias pragma after all.