arch: arm: enable lazy state preservation at boot in FP sharing mode

In FP Sharing mode we enable both automatic and lazy state
preservation of the FP context. This configuration improves
interrupt latency. Under this configuration the FP registers
will only be stacked when the thread is swapped out during
context-switch. Before jumping to main() we clear the FPCA
flag of the CONTROL register, so the FP context can be used
by the main thread.

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