[Bug other/69473] system identification macros not documented

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69473

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/onlinedocs/gcc-5.3.0/cpp/System-specific-Predefined-Macros.html#System-specific-Predefined-Macros

This manual, being for all systems and machines, cannot tell you what their
names are, but you can use cpp -dM to see them all. 

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