aarch64: run control rework

aarch64: run control rework

This patch contains a major overhaul of the target run control,
mainly for the sake of satisfying gdbs ideas of how a target
should respond to various control requests for the debugger.

The changes allow gdb a slightly better control on how cores
are stepped: a core can be single-stepped while
other cores remain halted or continue normal execution
until the single-stepped core halts again.

Also, on any halting event (user command or breakpoint) the
system is brought into a stable state with all cores halted
before the halt is signaled to the debugger.

This patch also transitions the target code to make use of the
new CTI abstraction instead of accessing CTI registers directly.

Change-Id: I8ddc9abb119e04580d671b57ee12240c3f5070a0
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3993
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>