target: try to reexamine even when polling fails

After intermittent connection failures or target power failures it
might be necessary to try reexamination even when polling fails. This
should make communication with Cortex-A targets more reliable.

This was runtime tested with stlink attached to an stm32l1 and an FTDI JTAG
adapter attached to an stm32f1 target.

Change-Id: I38c4db8124b7f4bbf53ddda53c13273449f49c15
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2721


Tested-by: jenkins
Reviewed-by: Felipe Balbi <balbi@ti.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Andreas Färber <afaerber@suse.de>