Add a flag to MIPSCPUClass in order to avoid needing to
replace mips_tcg_ops.do_transaction_failed.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20210227232519.222663-2-richard.henderson@linaro.org>