ARC: Fix failed assert with native GDB and child processes

There was a failed assertion in native GDB, when running hackbench with
child processes. That was happening because arc_linux_prepare_to_resume()
has been trying to obtain regcache for a new (child) process which is not
debugged by the GDB. To avoid a failure now we check for the last core of
the debugged lwp - if it is -1, then this is a new process and regcache is
not available.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>