gdb_server: run control fixes for vCont

gdb_server: run control fixes for vCont

this patch contains several changes to run control and state
handling together with gdb:
- graceful handling of target/gdb desync on resume, step and halt
- a default gdb-attach event executing the "halt" command, to meet gdb
  expectation of target state when it attaches
- call target_poll() after Ctrl-C command from gdb
- call target_poll() after resume and step through a vCont packet
- fix log message forwarding on vCont stepping, also move an aarch64
  log message from INFO to DEBUG level to prevent messing up the gdb
  console during source-line stepping
- fix oversight in vCont support that messes up breakpoint handling
  during stepping

Change-Id: Ic79db7c2b798a35283ff752e9b12475486a1f31a
Fixes: d301d8b42f0bfe67d76d6f340db6570cc71c876e
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4432
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>