ARC: Fix failed assert with native GDB and child processes

There was a failed assertion in native GDB, when debugging multiprocess
applications. 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, thus we don't have an address apce for it and regcache
is unavailable. Unfortunately I have found any failure-safe way to check if
regcache is available for a process and instead I have to check for
last_resume_kind which seems to represent well whether this is a new process
or an old one.

Unfortunately multiprocess/multithread debugging doesn't work for me with
native GDB, which seems to be caused by bugs in uClibc old threads debugging
library.

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