gdb_server: fake step if thread is not current rtos thread

gdb_server: fake step if thread is not current rtos thread

gdb assumes that a rtos can make any thread active at will in response
to a 'Hg' packet. It further assumes that it needs to step-over after
setting a breakpoint on frame #0 of any non-current thread. Both
assumptions are not valid for an actual rtos. We fake the step-over to
not trigger an internal error in gdb. See
https://sourceware.org/bugzilla/show_bug.cgi?id=22925 for details.

Change-Id: Ida60cd134033c1d58ada77b87fe664a58f61e2c0
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4448
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>