cpu-exec: Add "nochain" debug flag

Respect it to avoid linking TBs together.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

include/qemu/log.h

@@ -41,6 +41,7 @@ static inline bool qemu_log_enabled(void)
 #define LOG_UNIMP          (1 << 10)
 #define LOG_GUEST_ERROR    (1 << 11)
 #define CPU_LOG_MMU        (1 << 12)
+#define CPU_LOG_TB_NOCHAIN (1 << 13)
 
 /* Returns true if a bit is set in the current loglevel mask
  */