rtos: facilitate RTOS SMP handling

rtos: facilitate RTOS SMP handling

The RTOS handlers present OS threads to gdb but the openocd
target layer only knows about CPU cores (hardware threads).
This patch allows closing this gap inside the RTOS handler.

The default implementation just returns the current core, but
a RTOS handler can provide its own function that associates a
an OS thread with a core.

Change-Id: I12cafe50b38a38b28057bc5d3a708aa20bf60515
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3997
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Tested-by: jenkins