VFP fixes/cleanups for ARM11:
 * Save the required VFP registers on context switch. If the exception bit
   is set we need to save and restore the FPINST register, and if the fp2v
   bit is also set we need to save and restore FPINST2.
 * Move saving and restoring the floating point control registers to C.
 * Clear the fpexc exception and fp2v flags on a floating-point exception.
 * Signal a SIGFPE if the fpexc exception flag is set on an undefined
   instruction. This is how the ARM core signals to software there is a
   floating-point exception.