softmmu: fix for "after access" watchpoints

Watchpoints that should fire after the memory access
break an execution of the current block, try to
translate current instruction into the separate block,
which then causes debug interrupt.
But cpu_interrupt can't be called in such block when
icount is enabled, because interrupts muse be allowed
explicitly.
This patch sets CF_LAST_IO flag for retranslated block,
allowing interrupt request for the last instruction.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <163542169727.2127597.8141772572696627329.stgit@pasha-ThinkPad-X280>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>