target/mips: Extract decode_64bit_enabled() helper

Extract the decode_64bit_enabled() helper which detects
whether CPUs can run 64-bit instructions.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20241026175349.84523-2-philmd@linaro.org>
2 files changed