arc: show all registers on exception

The ESF contains register file contents including program
counter when the exception happened. If non-NULL and we
have ARC_EXCEPTION_DEBUG enabled, dump its contents to the
log stream.

Other arches do this already.

There is no need to read ERET, the ESF already contains the
interrupted PC value.

A future enhancement could create an option to additionally
push callee-saved register context into the ESF so it can
also be dumped out, but this patch does not address this.

A future enhancement could also convert the syscall
stack frame pointer passed to arch_syscall_oops() into
an ESF so that context of the failed system call can be
inferred.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>