arm: AArch64: Add support for nested exception handlers

In the current implementation both SPSR and ELR registers are saved with
the callee-saved registers and restored by the context-switch routine.
To support nested IRQs we have to save those on the stack when entering
and exiting from an ISR.

Since the values are now carried on the stack we can now add those to
the ESF and the initial stack and take care to restore them for new
threads using the new thread wrapper routine.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>