kernel: Move current_fp field out of z_kernel

The current_fp field in the z_kernel structure is only used
by 32-bit x86 (which does not support SMP). As such, it should
reside in the arch specific of section of _kernel.cpus[0].

This also changes the name of 'current_fp' to 'fpu_owner' to
be more consistent with other architectures.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>