[Bug target/104144] [12 Regression] build fails due to: Error: unknown architecture `armv9-a'

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104144

--- Comment #4 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
This sort of problem is going to keep occurring while we continue to have
separate distributions of GCC and binutils.  There's no way around the fact
that support for a new architecture in GCC needs an assembler that understands
the new architecture.  This is true for all architectures, not just Arm.

If a users asks for all the architectural libraries to be built, then I don't
see it as unreasonable to require an assembler that can support this as well,
so the issue becomes one of documentation, where we might as well just say that
the minimum requirements may be increased if you try to use anything beyond the
default configuration of the compiler.  Anything more is going to result in a
completely unwieldy mess of impenetrable dependencies.

I don't think there's a way to make the current multilib infrastructure ignore
specific sub-architectures.  t-multilib is already extremely complex due to the
need to reduce the number of variants to something just about tractable for the
build system; adding yet more complexity to it would make it almost impossible
to manage.

I guess it might be possible to make the multilib configure machinery rip out
variants that fail during configure, but you'd still need to deal with the
mappings and decide what to do if the compiler needed an unbuilt library
version.  Dropping back to the default multilib would often be completely wrong
as it might be an ABI change.