[Bug 204125] New: FTBFS on ppc64 big endian and gcc9 because of -mcall-aixdesc and missing __linux__

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https://bugzilla.kernel.org/show_bug.cgi?id=204125

            Bug ID: 204125
           Summary: FTBFS on ppc64 big endian and gcc9 because of
                    -mcall-aixdesc and missing __linux__
           Product: Platform Specific/Hardware
           Version: 2.5
    Kernel Version: any
          Hardware: PPC-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: PPC-64
          Assignee: platform_ppc-64@kernel-bugs.osdl.org
          Reporter: linux@octaforge.org
        Regression: No

On ppc64 big endian, the kernel builds with `-mcall-aixdesc` which since gcc
9.x removes `__linux__` from the list of macros being defined. This behavior is
supposed to be more correct (as it's in this case nothing but a hack, the flag
should apparently only be used when building for AIX) but sadly it breaks build
since several things within the tree rely on `__linux__` being defined and
`#ifdef` some of their code based on said macro.

Just removing `-mcall-aixdesc` (and using just `-mabi=elfv1`) is however not
enough, as that instead causes countless undefined references to just about
every symbol when linking `vmlinux`. It would seem that `-mcall-aixdesc`
changes the way symbols are declared in a way that is not expected.

Little endian is not affected because that one uses `-mabi=elfv2` exclusively.

For now I worked around it in my distro by explicitly adding `-D__linux__` in
the kbuild where `-mcall-aixdesc` is inserted into flags, and it works, but
that's obviously just a workaround.

I'm not sure what the proper fix would be.

Also, is there any reason not to allow an ELFv2 kernel to be built on big
endian? There are some supposed performance benefits, and ELFv2 itself supports
either endianness. The current kbuild logic pretty much forces ELFv1 for big
endian and ELFv2 for little endian.

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