gdb_server: fix HW thread status at gdb attach

gdb_server: fix HW thread status at gdb attach

At gdb attach, the event TARGET_EVENT_GDB_ATTACH is in charge of
halting the target.
For HW thread, rtos_update_threads() should be called after the
event to detect and record the new 'halted' status. Instead it is
called immediately before the event, thus reading the status
before the halt.

Move after the event the call to rtos_update_threads().

Change-Id: Iab3480ea0f5283ed6580f0f6c11200083197d1e9
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5983
Tested-by: jenkins