arm: cortex_m: select by default FP sharing mode when using the FPU

For applications that make use of the FPU in cortex m,
we enforce the FPU sharing registers mode, because the
compiler, under certain optimization regimes, may use
FP instructions and create FP context in any thread,
so the unshared registers mode is not practically
supported.

In addition to that we force FPU_SHARING to depend on
MULTITHREADING, as FPU sharing mode does not make sense
outside the normal multi-threaded builds.

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