arm float: Add Kconfig options

Adds Kconfig options CPU_HAS_FPU, FLOAT and FP_SHARING for the arm
architecture.

NOTE: All SOCs in the MK64F12 family have an FPU so that makes it a
convenient location to enable the hidden CPU_HAS_FPU option.

Change-Id: I71771d24f20f52079314bb8db9bf8a0aa827ab41
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>