UPSTREAM: arm64: reduce stack use in irq_handler

The code for switching to irq_stack stores three pieces of information on
the stack, fp+lr, as a fake stack frame (that lets us walk back onto the
interrupted tasks stack frame), and the address of the struct pt_regs that
contains the register values from kernel entry. (which dump_backtrace()
will print in any stack trace).

To reduce this, we store fp, and the pointer to the struct pt_regs.
unwind_frame() can recognise this as the irq_stack dummy frame, (as it only
appears at the top of the irq_stack), and use the struct pt_regs values
to find the missing interrupted link-register.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Bug: 30369029
Patchset: rework-pagetable

(cherry picked from commit 971c67ce37cfeeaf560e792a2c3bc21d8b67163a)
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
Change-Id: I84cbb04857a441083d331e875c3e228d24ec2276