ARC: Fix KSTK_ESP()

/proc/<pid>/maps currently don't annotate stack vma with "[stack]"
This is because KSTK_ESP ie expected to return usermode SP of tsk while
currently it returns the kernel mode SP of a sleeping tsk.

While the fix is trivial, we also need to adjust the ARC kernel stack
unwinder to not use KSTK_SP and friends any more.

Cc: <stable@vger.kernel.org>
Reported-and-suggested-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>