[Bug target/122414] OS_MISSING_POWERPC64 might be undefined for netbsd

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122414

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The macro is documented in XLC documentation since this is compatibility for
XLC:
https://www.ibm.com/docs/en/xl-c-and-cpp-aix/13.1.0?topic=features-macros-related-architecture-settings