arch: arm: Add code for swapping threads between secure and non-secure

This adds code to swap_helper.S which does special handling of LR when
the interrupt came from secure. The LR value is stored to memory, and
put back into LR when swapping back to the relevant thread.

Also, add special handling of FP state when switching from secure to
non-secure, since we don't know whether the original non-secure thread
(which called a secure service) was using FP registers, so we always
store them, just in case.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>