Deprecate TileGX port

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