[Bug libstdc++/107008] New: Combine config/os/*/error_constants.h into one file

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107008

            Bug ID: 107008
           Summary: Combine config/os/*/error_constants.h into one file
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

There's no reason to have multiple files and select one for the target:

config/os/djgpp/error_constants.h
config/os/generic/error_constants.h
config/os/mingw32-w64/error_constants.h
config/os/mingw32/error_constants.h

We can just ensure the generic one has a #ifdef for every errno macro that
isn't present on all targets, and use that everywhere.

Once we only have one error_constants.h file for all targets, we can also make
it work for freestanding like so:

#if _GLIBCXX_HOSTED
#include <cerrno>
namespace std {
  enum class errc
    {
      address_family_not_supported =            EAFNOSUPPORT,
      address_in_use =                          EADDRINUSE,    
      // ...
    };
}
#else
namespace std {
  // For freestanding we have no <errno.h> but also no errors from the OS,
  // so we can just make up our own values for the library's purposes.
  enum class errc
    {
      address_family_not_supported = 1,
      address_in_use, 
      // ...
    };
}
#endif


This will allow a freestanding <charconv> to use std::errc.