[Bug target/115289] PPC: Document the machine constraint 'Y' and make it non-internal

[segher at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115289

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
This needs backports all the way back to GCC 10 (well, as far back as we can
go, anyway :-) ).