[Bug c/106672] support Apple's old __private_extern__ keyword

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106672

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The macro seems appropriate for old code relying on a deprecated feature. I
don't think GCC should support this.