Deprecate TileGX port

Deprecate our TileGX target support:
 * we have no active maintainer for it
 * it has had essentially no contributions (other than tree-wide cleanups
   and similar) since it was first added
 * the Linux kernel dropped support in 2018, as has glibc

Note the deprecation in the manual, but don't try to print a warning
when QEMU runs -- printing unsuppressable messages is more obtrusive
for linux-user mode than it would be for system-emulation mode, and
it doesn't seem worth trying to invent a new suppressible-error
system for linux-user just for this.

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>
Message-id: 20200619154831.26319-1-peter.maydell@linaro.org

docs/system/deprecated.rst

@@ -404,6 +404,17 @@ The above, converted to the current supported format::
 
   json:{"file.driver":"rbd", "file.pool":"rbd", "file.image":"name"}
 
+linux-user mode CPUs
+--------------------
+
+``tilegx`` CPUs (since 5.1.0)
+'''''''''''''''''''''''''''''
+
+The ``tilegx`` guest CPU support (which was only implemented in
+linux-user mode) is deprecated and will be removed in a future version
+of QEMU. Support for this CPU was removed from the upstream Linux
+kernel in 2018, and has also been dropped from glibc.
+
 Related binaries
 ----------------