accel/tcg: Move TLB_WATCHPOINT to TLB_SLOW_FLAGS_MASK

This frees up one bit of the primary tlb flags without
impacting the TLB_NOTDIRTY logic.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>