ARM: unwind: support unwinding across multiple stacks

Implement support in the unwinder for dealing with multiple stacks.
This will be needed once we add support for IRQ stacks, or for the
overflow stack used by the vmap'ed stacks code.

This involves tracking the unwind opcodes that either update the virtual
stack pointer from another virtual register, or perform an explicit
subtract on the virtual stack pointer, and updating the low and high
bounds that we use to sanitize the stack pointer accordingly.

Signed-off-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Arnd Bergmann <[email protected]>
Acked-by: Linus Walleij <[email protected]>
Tested-by: Keith Packard <[email protected]>
Tested-by: Marc Zyngier <[email protected]>
Tested-by: Vladimir Murzin <[email protected]> # ARMv7M

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