Under FP shared registers mode (CONFIG_FP_SHARING=y),
a thread's user_options flag is checked during swap and
during stack fail check. Therefore, in k_float_disable()
we want to ensure that a thread won't be swapped-out with
K_FP_REGS flag cleared but still FP-active (CONTROL.FPCA
being not zero). To ensure that we temporarily disable
interrupts.
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>