debug: fail to show active cpu stacks [1/1]

debug: fail to show active cpu stacks [1/1]

PD#SWPL-4718

Problem:
Will have "PC=0, LR=0" issue when try to
show active cpu stacks via sysrq

Solution:
Save the addr of current interrupted task's context to the
interrupt stack first, so that we can easily obtain pt_regs
later

Verify:
Locally on Ampere

Change-Id: I1cc5095e0ec356bed90f76cdf9af1b2617ce7834
Signed-off-by: Jiamin Ma <jiamin.ma@amlogic.com>