target/arm: deprecate the pxa2xx CPUs and iwMMXt emulation

target/arm: deprecate the pxa2xx CPUs and iwMMXt emulation

The pxa2xx CPUs are now only useful with user-mode emulation, because
we dropped all the machine types that used them in 9.2.  (Technically
you could alse use "-cpu pxa270" with a board model like versatilepb
which doesn't sanity-check the CPU type, but that has never been a
supported config.)

To use them (or iwMMXt emulation) with QEMU user-mode you would need
to explicitly select them with the -cpu option or the QEMU_CPU
environment variable.  A google search finds no examples of anybody
doing this in the last decade; I don't believe the GCC folks are
using QEMU to test their iwMMXt codegen either.  In fact, GCC is in
the process of dropping support for iwMMXT entirely.

The iwMMXt emulation is thousands of lines of code in QEMU, and
is now the only bit of Arm insn decode which doesn't use decodetree.
We have no way to test or validate changes to it. This code is
just dead weight that is almost certainly not being used by anybody.
Mark it as deprecated.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250127112715.2936555-2-peter.maydell@linaro.org