gdb_server: avoid gdb server for virtual targets

gdb_server: avoid gdb server for virtual targets

Virtual targets, like mem_ap, do not or cannot implement the required
functionality to accept a GDB connection. In the case of mem_ap, the
method get_gdb_reg_list() is missing and a following connection from
gdb causes OpenOCD to segfault.
OpenOCD opens a GDB port for each target; it's always possible to
connect, by mistake, GDB to one virtual target.

Add a method to check if the target supports GDB connections (for the
moment just checking if get_gdb_reg_list is implemented).
Skip opening a gdb server for every targets that don't support GDB
connections.

Change-Id: Ia439a43efe1a9adbb1771cd9d252db8ffa32eb9d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4676
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>