linux-user/mips: Select Octeon68XX CPU for Octeon binaries

linux-user/mips: Select Octeon68XX CPU for Octeon binaries

The Octeon68XX CPU is available since commit 9a6046a655
("target/mips: introduce Cavium Octeon CPU model").

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1722
Reported-by: Johnathan Hữu Trí <nhtri2003@gmail.com>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240814133928.6746-3-philmd@linaro.org>