arm: cortex_m: force FP context stacking by default

For the standard multi-theading builds, we will
enforce FP context stacking only when FPU_SHARING
is set. For the single-threading use case we enable
context stacking by default.

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