arch: arm: fp sharing: save & restore FP registers in context-switch

When in Sharing Floating Point Services mode we want to
dynamically save and restore the FP registers in thread
context switch, depending on whether the swapped-in and
swapped-out threads are currently using the FP registers.
This commit adds this functionality to the ARM context
switch mechanism. The logic consists of inspecting the
corresponding status flag (present in thread.arch.mode)
to decide whether to save or restore the FP registers.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>