Deprecate 32 bit big-endian MIPS

Deprecate 32 bit big-endian MIPS

It's becoming harder to maintain a cross-compiler to test this host
architecture as the old stable Debian 10 ("Buster") moved into LTS
which supports fewer architectures. For now:

  - mark it's deprecation in the docs
  - downgrade the containers to build TCG tests only
  - drop the cross builds from our CI

Users with an appropriate toolchain and user-space can still take
their chances building it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-22-alex.bennee@linaro.org>