[Bug libstdc++/104883] <system_error> should define all std::errc enumerators

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104883

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
>From PR 1007008 comment 2:

-- >8 --
We could do something like this for each enumerator:

#ifdef EFOO
   foo = EFOO,
#else
   foo = _GLIBCXX_ERRC_ORIGIN + __LINE__,
#endif


Where _GLIBCXX_ERRC_ORIGIN is some value intended to be higher than the highest
errno value on the OS. We only need fewer than 100 errc enumerators so
INT_MAX/2 might be OK.

We can use #line to set the __LINE__ to a fixed value in the file, so that the
constants don't change if we add/remove lines before the errc definition.
-- >8 --

We can use a default _GLIBCXX_ERRC_ORIGIN value, and allow it to be overridden
in config/os/*/os_defines.h if the default doesn't work.