Deprecate lm32 port

Deprecate our lm32 target support. Michael Walle (former lm32 maintainer)
suggested that we do this in 2019:
 https://www.mail-archive.com/qemu-devel@nongnu.org/msg605024.html


because the only public user of the architecture is the many-years-dead
milkymist project. (The Linux port to lm32 was never merged upstream.)

In commit 4b4d96c7 (March 2020) we marked it as 'orphan' in
the MAINTAINERS file, but didn't officially deprecate it. Mark it
deprecated now, with the intention of removing it from QEMU in
mid-2021 before the 6.1 release.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Michael Walle <michael@walle.cc>
Message-id: 20200827113259.25064-1-peter.maydell@linaro.org

docs/system/deprecated.rst

@@ -408,6 +408,14 @@ The above, converted to the current supported format::
 linux-user mode CPUs
 --------------------
 
+``lm32`` CPUs (since 5.2.0)
+'''''''''''''''''''''''''''
+
+The ``lm32`` guest CPU support is deprecated and will be removed in
+a future version of QEMU. The only public user of this architecture
+was the milkymist project, which has been dead for years; there was
+never an upstream Linux port.
+
 ``unicore32`` CPUs (since 5.2.0)
 ''''''''''''''''''''''''''''''''